Most professional technical ebooks are in .chm format these days, that's microsoft's compiled html format. Wow, look at that, its like a zipped directory full of html files and graphics. How wonderful to have another bleeding file format.
In any case, its been reversed by seekers, so no worries about decompiling chm. You can do it with a little seeking, you'll get the tools if you need to do this. Personally, I can't be bothered yet (2003-12-04 02:56:37 EST).
Anyhow, ebooks these days come in chm format a lot, especially if you are interested in microsoft software. There are some great books that are for the technical layman in this format.
As well, you can get ebooks in pdf format, that many people don't like at all, and I understand why. The reader for it is disgustingly bloated -- but there are some great books in this format, so don't discount it on account of the tools available today to read it. You could always make a better reader, or wait until someone else does. On the topic of pdf's, don't use acrobat5 on windows, rather get acrobat4 -- its a LOT faster.
Okay, so we have pdf + chm... there is also lit -- a microsoft format if i remember, though I haven't used it to date -- I'm pretty sure most of the books that come in this format are non-technical in nature, and I'd rather read a paperback novel than one online, especially since real life libraries exist.
Then of course there is html, which, depending on the formatting of the pages can be either a joy to read, or a pain in the rear end.
Lastly there are scanned images. You'll sometimes just get a zip of scanned images, but usually these days they will be assembled into a pdf. How quaint.
Sometimes people get these ebooks from the cds that come with the books, sometimes they are available online for pay by publishers and then get disseminated to the public at large, and sometimes people lovingly scan in pages and then use character recognition software (OCR) to translate it into text. And then there are the really commited folk who type in the books by hand.
Now how to actually find these ebooks? Well the best bet is to use a multiple pronged approach = ask your friends + usenet binaries + file sharing tools + searching the web with clever keywords + irc file sharing.
It's all there for the reading. See if you can find an online version of Vernor Vinge's True Names. Now that's a great read.
More on the mechanics of finding ebooks later.
One more thing = dont become a zombie book downloader who collects books and never reads them. Now that is just plain dumb. I see it time and time again, and its a sad state of affairs.