Use a typeface that is appropriate for your readers
There are many thousands of different typefaces available. If you are planning to supply your own finished layout, your main consideration with respect to choosing typefaces for a book should be legibility.
Some typefaces are easier to read in block text than others. The most commonly used kind of typeface used for block text in books is a serif typeface such as Times or Garamond. Headings are often set in a contrasting sans-serif typeface such as Arial or Futura.
When choosing a size for your text, consider the readership. Young adults may have no trouble reading 8pt type, but an older audience may find 12pt type much more comfortable to read. Young children may also need special consideration. Remember that you want people to finish reading your book!
Of course, larger type means more pages, which in turn increases the cost of printing, but it will also increase the physical substance of your book. A 250-page book with text that can easily be read may be easier to sell than 150-page book with text so small that every page is a struggle. If you only print 50 copies, the extra pages are hardly going to impact on the overall cost of your book.