There aren't any true reference books just for small badges. Various reference books but authors like David Littlejohn will sometimes cover small badges that fall under the subject that the book is about but you have to wade through lots of other info on uniforms etc to maybe find one paragraph on a badge. I do know a collector who is currently writing a book on small badges, but when it will be ready I do not know.
Hüsken published his last of what is really just a catalogue of small badges which is often used as a guide by collectors back in 2010. However, the entries are not always correct, the reverses of the badges/pin (sometimes the most important part in identification) are not shown & his price guides are way off (often too high or too low). And, there are known fakes shown as well...
As Joe says, you are probably better off sticking with forums as then hopefully you can get opinions from several sources.
Here is a scan of the cover & a sample page from the Hüsken catalogue if you are interested in it. It's over 500 pages but will set you back about £50.