Basic Book Design/Spaces Between Sentences
Type one space between sentences.
LaTeX automatically puts in a wider space between sentences. One LaTeX manual says that this is traditional for English and American typesetters, and that French typesetters don't use the wider space between sentences. The Chicago Manual of Style doesn't say anything about this issue.
I.e., the "keep out of trouble" rule is one space. I'm not certain of the traditional rule. The "word processor rule" is easy: Microsoft Word can't put wider spaces between sentences.