It's possible to convert PDF files to text based files. As mentioned, Calibre can do so, and also Mobipocket Creator Pro (when using this, you'll need to remember to save as HTML and then later have Calibre convert it - I don't know if this will improve quality, but it's a different option at least). When converting from PDF, you will usually have to accept minor errors in the formatting. It's far from ideal, most any other ebook format is a better starting point, and you will definitely have problems with for example graphs or code samples.
If you are from the USA, I understand that a large number of libraries provide ebook on loan - and that it might in some cases be possible to loan from libraries in other states (perhaps given a fee). If you are from another country, you'd better contact your local libraries. My library in Denmark do offer a good handful of ebooks, but none that interests me, unfortunately
