You can use a tool like
http://www.pdfscissors.com/ (web tool, no installation needed) to remove margins and, optionally, cut up pages (if the text is still too small, use landscape mode.). I do it all the time with technical papers/books, the procedure makes them very readable on my Kindle 3.
Of course, it would be better if you had LaTeX (or docbook, etc) source for the books, in that case you could generate PDF formatted specifically for the small screen. I do that with papers from arxiv.org with very good results.