Are you sure you want to contribute? 
Read Help:Why contribute? first.
Does the book or a similar one already exist? 
- It is easier to build on existing books than to start a new one.
- Check on the bookshelves if similar books exist, or if your content could be part of an existing wikibook.
Are you willing to support the new book? 
There exist many books that have a great introduction paragraph, but not more. Are you willing to write a substantial part of the book by yourself?
Be bold, Start the book 
If your answers are: YES, YES, NO, YES, then start the book!
Point of Sale Systems reside in a space in which everything goes wrong, at the most inopportune moment. Murphy is quite at home here. This strange reality has led to the fact that instead of attracting the nerdy software types, this industry tends to bring in the battle hardened types. This has led to the creation of sturdy and rugged applications that appear to be created for the specific task of surviving an unforgiving environment. The fact that all the data that is being handled is generally highly classified, goes a long way to reinforce the notion that users will generally accept less sophisticated applications in favour of what they see as tried and tested. Ugly as sin, but pays the rent type applications.
We often find that our public has come to accept far less that than they deserve, simply because they are afraid of what may happen if they change, or that the information that lies in the hands of their IT guys is so sensitive that one needs to lock him into a room, and never let him out. (figuratively in the form of some contract I mean). This always, and I mean always leads to a very predictable outcome.
1.) The software tends to run ok, with lots of bugs, that everyone in the organisation has learned to live with. They roll up their eyes and say, "Software problems, whats new?". 2.) The software begins to get frayed, around the edges, and it is clear that the client is paying too much, for too little. At least he thinks so.
Technically start the Book 
Read the naming policy first on how to arrange and name your book. Then make the contents/cover page. Name the title what you would like the book to be called, choose a short and descriptive title, but not abbreviations. Create the page the way you want it, and save it.
Show the book to the public 
Next, you have to make the book available to other users. Of course, people can see it on Recent Changes, but its visibility on that list is not permanent, so you need to put it in a bookshelf. Go to the bookshelves, and put your book into a bookshelf you choose. If you are not sure, choose the Miscellaneous bookshelf, and another editor will put it elsewhere if necessary.
Once you have gone to the page of the bookshelf you want to add your book to, add your book in the correct category (if applicable) in alphabetical order. Save that page and go back to the Main Page. To get your book by typing it in the search box or URL. Create all of the pages within that textbook as "Textbooktitle/Whateverthepageis" to avoid problems with other textbooks. See Wikibooks:Naming policy for more.
You might also want to add the book to the list of new Wikibooks that appears on the main page. To get there, click on this link, or click "(edit template)" on the main page. Make a link to your page at the front, and follow the rest of the directions you see at the bottom of the source. Save that page.
Note: If you are a new user, you cannot place your book on the bookshelf by yourself. Once you add your book to the list new Wikibooks, then someone will put it there for you. This is a protection Wikibooks needs to counter vandalism.
Adding Wikibook suggestions 
If, on the other hand, you think you have an idea for a new Wikibook but you are not completely sure on how far you want to go with it, you can add a suggested Wikibook on most of the bookshelves instead. Most bookshelves have a list of suggested Wikibooks, and feel free to add suggestions. If you are feeling inspired to write some material, you might even want to see what others have suggested and create one of those suggested wikibooks instead.
Another very appropriate place to try and see if other people would be willing to help out on a new Wikibooks is the Staff Lounge. Many regular contributors to Wikibooks read this page periodically, so you can get some suggestions for how your book idea can be put together, or perhaps find some other people who might be interested in helping you out in putting the book together. This is also a very good place to suggest new titles for Wikibooks that perhaps you might not have the time to complete yourself.
Take a look at other books 
You can learn how to create a good book and find new ideas by analysing existing ones. Wikibooks don't have strict policies determining shape of a book so don't be confused if you find books that are designed completely different. Generally, it's a good idea to look up some featured books like How To Assemble A Desktop PC.
Additional elements 
Creating printable books 
If you want people to read your book as continuous text, in a PDF file etc., it is worthwhile maintaining a "print version" from the start. See Help:Print versions.
You can create "Foreword" or "Introduction" chapter explaining what is the scope of the book and how to read it.
It's likely that other people may edit your book. There should be a page listing most important contributors to the book. It can be named "Authors". It's a good practise to create a manual of style for the book, explaining how the book should be written, which templates are used etc.
Stage of development 
You can mark which chapters are finished using development stages marks. You can also show how much of your book is ready in bookshelves where you book is placed.