"Guppies, Mollies, Platys, and Other Live-bearers. A Complete Pet Owner's Manual" by Harro Hieronimus.
This book contains a chapter with Advice for Buying, Maintenance and Care (EXCELLENT!! has a table for a maintenance schedule that I wish I could follow better!), The Proper Diet, and Diseases, Breeding, and Varieties.

Another thing that I love about this book, is the last chapter on Popular Live-bearers and their care. This section goes through each species, and provides further information, it also  narrows the temperature, pH, etc. closer to the "optimal" than many other sources I have found.

Last but not least, is, "Guppies" by Wilfred L. Whitern.  This book has some excellent photos, and information as well.  It also has more genetic information on inheritance in the guppy.

So those are my top three picks.

Anyone, and everyone is invited to add their favorites to this entry.

Happy Reading!