Yes, there are books for agate identification; also web sites
devoted to it. My suggestion is to check these two sites
The slab which you purchased is similar to several pieces which I
have seen from New Mexico, although that is no guarantee your piece
is from that state. Agates are essentially infinite in variety, as a
quick tour of either of the above sites will illustrate. Sadly, even
though many varieties are relatively (or even absolutely) rare,
agates scarcely ever command huge prices. Those which are reasonably
expensive remain bargains when compared to many other gems and
minerals used in jewelry.
You found a good one!
Keep on rockin’
Small Wonders Lapidary