Here is a few more links that might be of help.

There used to be a science fiction specific book swap website. The one I remember does not appear to be active anymore. I did find this:

paperback Swap    http://www.paperbackswap.com/

For Fem SF there's this:

SFBookswap           http://sfbookswap.wordpress.com/

Through my local library I can get access to ebooks at NetLibrary (check with your local library about this).

DailyLit delevers books to your email in small daily chunks, http://www.dailylit.com/.

If you don't mind reading online there are these sites:

Page by Page Books   http://www.pagebypagebooks.com/

The Literature Page     http://www.literaturepage.com/

LibriVox                           http://librivox.org/

BookView Cafe              http://www.bookviewcafe.com/

For inventorying and keeping track of your personal library:

LibraryThing              http://www.librarything.com/

Bibliophil.org             http://bibliophil.org/default.php

Something fun to try:

BookCrossing         http://www.bookcrossing.com/