GURPS Books are pretty good, although they tend to be very "realistic" - whatever that means when you speak about Magic and Spaceships.
However, their crunch is very easy to convert - 1 Round in GURPS is exactly one second, and modifiers to skills/attributes pretty much scale 1 to one to DnD - GURPS uses 3d6 range instead of d20.
Apart from that, the Books are pretty crunchlight, and really well thought out/ researched. Especially good: Vehicles (if you want to build Vehicles that behave realistically, even though they come from a Steampunk setting or whatever), Time Travel, Illuninati (for playing in a world where Conspiracy theories are true. At least some of them. Fnord.) and basically whatever topic you are interested in. Really, these Books ar A-awesome.