Most e-books are available as PDF-files and can be read free of charge with Acrobat Reader. Digital Editions may in some cases also be required.
More information about the readers is available at the e-book suppliers websites.
Special formats
If your reader requires special formats,
there are progams that can convert the PDF-files for you.
The MobileRead wiki: http://wiki.mobileread.com/wiki/E-book_conversion includes an extensive list of progams and what formats they can convert your files into. The programs are available for Windows, Mac and Linux. Some of them are free to use, but not all.
Special E-book Readers
A special online reader is only required for Ebrary. Download instructions for this reader are given on the ebrary start page, under the heading help.