By searching for ebooks, one can usually find textbooks for free, but what options are they for those who want to do so legally?
What about old books that are still in print? I like to have electronic versions of textbooks. It doesn't seem reasonable to manually scan big tomes!
http://safari.oreilly.com is pretty good - look at a wide range of books online (not just OReilly), with a subscription option to get chapter downloads in PDF.
One publisher that does this is Pragmatic Programmers. They give you the option of purchasing ebook-only or paper & ebook editions. They also email you when the books get updated so you can d/l a new pdf.
Some books are published online under permissive (Creative Commons, GFDL etc.) licenses.
Examples are Real World Haskell, Learn You a Haskell, The Art of Unix Programming, TeX by Topic and many more. I like this trend, often prefer such books and recommend them if they are good.
My company has a subscription to Books24x7 which has online copies of any book I've ever looked for, and with a great search capability. I don't know what the cost would be for a personal subscription. I believe they have an option to create and download PDF subsets.
If you're looking for ebooks about .NET, Paul Sheriff has written several books that he self publishes as ebooks.
Check out your local library. Through my library I can access e-books, safari online, abstracts, and of course hard copy books from many participating libraries. best part all the above is free.