Applied Cryptography is a good book. I first read it more than 15 years ago :). Schneier actually designed the CSPRNG that is used by OS X. It's called Yarrow a

Applied Cryptography is a good book. I first read it more than 15 years ago :). Schneier actually designed the CSPRNG that is used by OS X. It's called Yarrow and is SHA based. It does more or less what you suggest, but with many subtle improvements to mitigate attacks. It has features that you're unlikely to properly reproduce in user-space. Trying to improve its output by re-hashing in user-space is, even if done properly, unnecessary. If you screw something up and do it improperly you could easily reduce the quality of the generator. I'm not sure how your link is relevant. If there's not enough entropy on the system for the kernel, there's not enough for you either.