Classical Guitar 
The classical guitar has a historical development that can be traced back to the lute and vihuela music of the 15th century. This historical basis gives the classical guitar its technique and repertoire. Of all the guitar disciplines, it is the classical guitar which has the greatest amount of literature and music available to the student. One of the greatest of the past masters of classical guitar, Andres Segovia:
"The strongest advice I give to my pupils is to study music properly from the beginning to the end - like the career of a sergeant or a physician, it is the same. It is a shame that most guitarists are absolutely clean of this knowledge. My advice is to study music properly and not to omit any knowledge of music and not to be very impatient about giving concerts. He who is impatient mostly arrives at his goals late. Step by step is the only way"
Quote from Segovia! A 13-part series aired on National Public Radio. First aired April 1983 and produced by Larry Snitzler (Classical Guitarist) and hosted by Oscar Brand (Musicologist/Folk Guitarist).
Any guitarist who wishes to learn to read music should use classical works. Classical studies are designed to develop the student's sight-reading skills at the optimum speed. Classical guitarists use standard works to learn from; especially the works of Fernando Sor (1778–1839), Mauro Giuliani (1781-1829), Matteo Carcassi (1792-1853) and Francisco Tárrega (1852–1909). For complete beginners, the author Frederick Noad has provided the books: Solo Guitar Playing One and Two. These two books by Noad assumes that the student has no previous experience of reading music and the lessons have been carefully arranged with this in mind.
The Parts Of The Classical Guitar 
An essential item for all guitarists. It allows the left leg to be raised when the student is sitting. The early classical players explored all the various playing positions and the seated position with a foot-stool was found to give the greatest access to the fretboard and allowed barres to be played with ease.
A music stand allows the guitarist to maintain his playing position and ensures that the music is at eye level. Despite its usefulness, a music stand is usually one of the last items thats guitarists buy. Many guitarists are quite happy with placing a book on the edge of the bed or on the floor. This means that the player is looking down at the music and is constantly having to adjust his head and body position to scan the neck when changing chords. Having the music on a stand at eye level means that a guitarist only has to glance at the neck to ensure correct finger placement. Whether you are a rock guitarist who stands up to play or a classical guitarist who sits; the simple act of setting up a music stand will make learning more comfortable and quicker.
A metronome is the ideal tool for improving one's timing. Ideally you should buy a traditional wooden metronome with a wind-up mechanism. Though computers offer a digital version, the wooden metronome provides an organic "click" sound which is pleasing to play along with for extended periods. Be aware that the metronome highlights your timing errors. Always set the metronome to its slowest speed and then play. Only after mastering a piece of music at the metronome's slowest speed should you increase the tempo.
The classical guitarist plays without a plectrum. Pianists can play the notes of a simple C major chord so that they all sound at the same time. This can only be achieved on the guitar with the right-hand finger nails. A classical guitarist will place his right-hand finger nails on the notes of a chord and will give a slight twist; almost as though he were twisting the lid of a jar. This technique allows all the notes of the chord to sound at the same time. Therefore it is essential that your nails should be considered. Professional guitarists in all genres use nail hardener; this can be found in any chemist. The nails should be filed with a fine-grade nail file.
Do not lean your guitar against walls or furniture because this is the most common cause of neck breakage during accidents. A guitar stand will keep your guitar out of the case and ready to play. Buy the best stand available on the market since all the better guitar stands have soft covers on any edges that will protect your guitar if the stand is knocked over. Some guitarists hang their guitars on the wall in the same way that you see guitars displayed in a music shop. However a guitar stand is still required for gigs.
Learning To Read Music 
Learning to read music involves mastering one note at a time. Its very common to find second-hand music books with letters hand-written above each note. The idea that you can learn to read music by deciphering a single piece of music that you like should be discarded. Its only natural to want to learn something you are interested in but you cannot master music notation in this way.
Learning to read music in the early stages involves only the open strings. The idea is to introduce the notation for the fretted notes after the notation for the open strings has been mastered. The first day will involve starting with the notation for the open high e (thinnest string), followed by the B string, then the G and so on. Once a student can read the notation for the open strings comfortably; the notation for the first three frets of the high e is introduced, then the first three frets of the B string and so on. You will not find a good music book about learning to read guitar music that doesn't follow this set method. However playing open strings hardly makes for a great song though once the fretted notes are introduced extended melodic phrases can be played.
At a more advanced stage of reading music the student is advised that familiarity with their own sheet music collection can actually be a problem. It is not unusual to find that a guitarist who has used the same sheet music to learn a piece suddenly struggles when the same piece is presented to him from another publication. Fonts, staff spacing and even elements as benign as the paper size and colour can cause difficulties in sight-reading. The student is advised to vary their sight-reading by using different publications.
Standard Works 
Please note that the following list of works will not teach you how to read music. For that purpose you need to use the three Noad books: Playing the Guitar and Solo Guitar Playing 1 and 2 which have proved very popular with teachers and self-taught guitarists.
25 Etudes op. 60 - Matteo Carcassi
This work contains a series of studies designed for intermediate players. Consisting of Roamntic arpeggio pieces with scale and positional studies; this classic work has served generations of players with its inventiveness and melodies. Though didactic in purpose; Carcassi has provided audience-delighting studies that form part of the repertoire of many professional and amateur players.
Espanoleta - Gaspar Sanz (Pujol Transcription)
The Espanoleta by Sanz is deservedley famous. Its simplicity and beauty is accessible to the early-stage guitarist who can read music in the first position. Its common to find a transcription of the Espanoleta in many sheet music compilations and tuition books; including Playing The Guitar by Frederick Noad. The Pujol transcription is a simpler arrangement than the Noad transcription and provides the guitarist with practice in recreating Baroque ornamentation.
The Guitarist's Hour Books 1, 2, and 3 (Walter Gotze)
These collections of studies for beginners consists of pieces by Sor, Carulli, Aguado and many others. They are designed to encourage the student to observe note duration as well as providing an introduction to the works of Sor and Carulli. These progressively graded studies are ideal for metronome pratice. Though most of these studies can be found in other compilations; the choice and arrangement of the studies in all three Guitarist's Hour books has proved popular with teachers as well as self-taught players.
Ways To Improve Interpretation 
For many guitarists playing along to records is an ideal way to improve your playing. At the end of Solo Guitar Playing One is a short binary piece called "Adelita" by Tarrega. Julian Bream has recorded "Adelita" and it is recommended that after learning "Adelita" that the student should play along to the Bream recording. It will improve your own interpretation of the piece and will impart a deeper understanding of the popularity of the Salon style of Chopin that Tarrega uses in this composition and that Bream captures in his interpretation.
The famous "Leyenda" by Albeniz is a technical challenge for any guitarist. Transcribed for guitar from the original piano work it has become part of the classical guitar's repertoire. On film we have two master classical guitarists performing "Leyenda": Segovia (filmed at the Alhambra Palace) and John Williams (Concert from Seville). Due to the popularity of the piece it regularly appears in sheet music compilations though it must be noted that original transcriptions of "Leyenda" are personal expressions of the transcriber's own technique. The two guitarists mentioned have chosen to adapt the piece to their own technique.