This is the 12th in the "Discworld" series, a British humor/fantasy bunch of books comparable in style to "Hitchhikers' Guide to the Galaxy," but it's only the setting (a flat world resting on the back of four elephants resting on the back of a turtle) that's consistent, not so much the characters, so it's not necessary to read the previous ones, though a couple of the jokes are running gags or references to the previous ones.
Why I'm bothering to review this is its picture of gods and how they work, and what this exemplifies about the treatment of the metaphysical in popular, fun fiction, and really, what good religion is to us in these modern times. Plus, there's a whose section of the book that takes place in a fictional version of Athens with a lot of fine jokes about ancient philosophy, a brand of humor that I've not seen in a whole lot of other places.
It's been posited before in fiction that gods get their power from the attention and belief of their believers. This is a flippant response to the obvious goofiness of why gods (modern or ancient) would have any interest in our belief, let alone worshiping and groveling and sacrificing lambs and such. Were I a god, all this fawning would make me pretty uncomfortable, but of course gods were invented in an age of crushing despots, where the god is super-king and often serves to reinforce the rule of the king.
Well, "Small Gods" explores that line of thought, with the premise that there's an entire nation (Omnia) devoted to one big god (Om), but the church has become so institutionalized (with a violent "Quisition" torturing and killing anyone even smelling of heresy, or anyone else that its leaders feel like), that in fact almost no one actually believes in Om any more, with the consequence that when Om decides to become an animal and visit the earth for a bit, instead of a big fiery bull or something, he becomes a tortoise, with the mentality and approximate abilities of a tortoise, and it's only when the SOLE believer left in the country, a low-level monk and gardener, happens to pop by, that Om is able to communicate with him, have some rational self-reflection, and do some minor god-like acts (e.g. lightning bolts on par with a bad static electric shock), and so he wants to use said believer as a vehicle to get his powers back, and so hilarity ensues.
So, what's the philosophical import? Well, first, religious history is filled with good stuff to make fun of, and however much or little you may think it applies to your modern belief system, that whole "I'm a jealous god and will kick your ass" thing from the old Testament and Greek mythology and such is pretty amusing. So, it sure is fun to play with such ideas, and my reading tendencies have favored this direction (e.g. Mike Carey's comic book series "Lucifer" is another example, as well as its parent series, Neil Gaiman's "The Sandman," as well as most of the rest of what Neil Gaiman's ever written).
...And it's not just the internal content of old belief systems that's strange and fun, but the relationship of belief to ordinary life, to technology, to politics, that makes for many a fine plot. I won't pretend that "Small Gods" has anything profound to say about morality (e.g. that you should not kill in the name of belief) or philosophy (that most of it is useless, except that one idea out of 100 that creates some new massive technological boom), but like any good philosophical fiction, it gets ya thinkin', which is, really, the best we can hope for, even from a source that claims to be serious, and moreover, the thinking is brought about in a way that is actually enjoyable, unlike most philosophical texts or more ham-handed and less reflective attempts at philosophical fiction (like, say "Brave New World"). Pratchett is working in the tradition of Voltaire ("Candide"), with the added benefit of the advance in years/thinking/distance from the time when religion dominated the earth. So, yes, I recommend the book.