They both have a lot of "how do I get a license" administrative trivia. I think that's a universal constant, and nothing can ever free us from that. Such is life. However, when you get into the actual "how do I drive without being arrested" section, they kind of . . . diverge.
For example, the Washington Driver's Handbook has a mini-table with speed limits. Arterials, as I remember, are 40mph. Residential roads are 25mph, except when they're 15mph. Freeways may or may not be 55mph.
The California Driver's Handbook says that residential roads are 25mph, and highways are probably 65mph. If they're not, they'll be marked. And, oh, by the way, it's probably safest to just go the speed of traffic.
The Washington Driver's Handbook gives hard fast rules. If it is foggy, go this fast, with this light configuration on. If it is raining, decrease speed limits by approximately this amount. Etc etc. And on and on.
The California Driver's Handbook (usually) gives guidelines. If you're approaching the top of a hill or the end of a sharp curve, and can't see what's ahead of you, slow down. If it's raining after a dry spell, the roads will be slippery, and so you should drive very carefully. There are, of course, some rules, but they're quite logical - "if you are a truck carrying explosive materials, stop before crossing railroad tracks." "If you cannot see anything, pull off to the side of the road until you can see things."
The most bizarre part comes when you get to the written test. Most tests, I've found, attempt to conflate "legality" and "safety" into one package. The California test doesn't *quite* come out and say "you might be safest by doing illegal things", but it comes damn near.
Example: One question says "Traffic is moving at 70mph on a highway. What is the maximum legal speed?" with options varying from "as fast as I want to go" to "65mph". The correct answer, of course, is 65mph.
The next question says "When merging onto a highway, what is the safest speed to be traveling?" with options varying from "65mph" to "the speed of traffic". The correct answer, of course, is "the speed of traffic".
Even if, you know, traffic is traveling at 70mph.
It's rather refreshing to have a government publication that - apparently - candidly admits that a large number of people break laws every day without spontaneously exploding or turning into pedophiles.
I'm not really going anywhere with this. I just found it interesting.
Unrelated but still vaguely on-topic, today I had to pass a Corvette doing 65mph in the fast lane (which regularly hits and exceeds 80mph). I mean, WTF. There weren't even any cars in front of it.