I'm currently writing a fantasy book series that has a plot that covers many years. If anybody wants to know what the book series is like, expect magic, spirits, insanity-causing substances, anything involving the number 7, and more.
Ps: For the record, it's still in the beta stage, so expect some stuff changed around.

Have you done Nanowrimo? If that counts I've written something like six or seven novels. The Nanowrimo.org site itself and the forum has some good resources, especially regarding world-building and research (though the former probably doesn't matter to you).

It's tricky, I usually write genre fiction, to advise you on non-fiction.

I began a long non-fiction book, but never finished it. It would have been similar to what you described. The only advice I can give - besides "stay interested" - is to have a very clearly-defined outline ready before you begin, and to have someone utterly unfamiliar with the subject matter read it when you're done, so as to catch any and all bits that need clarification.

http://www.fmwriters.com/community/dc/dcboard.php

I highly recommend this site for writers. They have an excellent number of writers. he chat room is really useful. Several published authors hang out in there. Most of them are primarily fiction writers, but there are some that write, or have written non-fiction, also.

Most important thing when writing non-fiction is that you have a plan about what you are going to write. And that you like the subject matter.

Depending on the plan it can be a lot of essays that fit together. Though in a book filled with essays those essays can be loosely related.