Best thing to do is just get out there and take photos and post some on forums like this.
As commented above John Shaw has done some useful reading and a new one by
Bryan Peterson of understanding exposure fame is supposed to be good.
I thought John Shaw's "Closeups in Nature" was OK, but a little bit dated. Macro has changed quite a bit since the book was written -many of us don't even own a tripod because the critters that we shoot won't sit still long enough to set one up...