By and large, Buffy novels are a horrible, soul eating experience. Buffy never has even a fair impression of a personality and says "like" about four million times a paragraph. Giles is there to set up jokes for Xander, which are always weirdly phrased. Spike speaks like Mary Poppins. Willow then saves the day with a Spell-Ex-Machina, which allows Buffy to break free/ find the Scooby Gang/ find the bad guy. Buffy destroys said bad guy. Book ends abruptly. One exception I can think of is the Chaos Bleeds novelisation, which managed to give everybody a personality, and, shockingly for a novel, got that Faith has more to her than the tough kid act, without over doing her vulnerability or letting the Scooby Gang see it.
Buffy and Philosophy... is good, but occasionally frustrating. I frequently found myself wanting to find the various writers, and yell at them my counter-arguements. However, any book that makes you yell at it can't have been a wasted experience.
The Watcher's Guides are shameless cash ins. They're glossy, purdy and have nice pictures, but the content is desperately shallow, and, in keeping with official merchandise, they refuse to acknowledge a single flaw has ever besmirched the production of a Buffy episode. Which is to be expected, but it's very difficult to take a book seriously after it's suggested that "I" in Team" is a crucial and pivotal episode to the Buffyverse. There are much, much better episode guides out there.
Joss Whedon: The Genius Behind Buffy suffers from the same problem. It's essentially a collection of quotes from Joss Whedon. Which is great, he's a really interesting guy, and very funny to read, but we learn nothing new. Everything he has said or done is treated with near-religious reverence. Nothing is ever questioned, and it's an extremely lightweight read. It's worth borrowing or reading for free, but it's a shame that such a naturally fascinating guy, with such an interesting life, is given such breezy treatment.
I've heard that the best guide by far is Dusted, but it's tough to get 'round these parts. Anybody?