Below are a collection of links and sources for book reviewing.

A very good overview of doing book reviews from someone who's been doing it professionally since 1973. Very good for the beginner reviewer or for someone doing a more casual review.
http://www.well.com/~ladyhawk/bookrevs.html

From the Book Critics Circle (the professional organization of bookrReviewers). This is probably the best overview of doing book reviews we've found. You can join for $40 a year after you have three reviews published:
http://www.bookcritics.org/?go=tips

John Updike's Rules for Book Reviewing:
http://bookcriticscircle.blogspot.com/2006/06/reviewing-101-john-updikes-rules.html#%3C/a

The New York Times 7 deadly words of book reviewing:
http://papercuts.blogs.nytimes.com/2008/03/25/seven-deadly-words-of-book-reviewing/

Also at the New York Times, Joe Queenan discusses using words like"astonishing:"
http://query.nytimes.com/gst/fullpage.html?res=9501E5D91230F932A15752C0A9619C8B63&scp=10&sq=Queenan&st=nyt

From the Internet Public Library:
http://www.ipl.org/div/farq/bookreportFARQ.html

If you have a place to review, this site has a number of publisher contacts for getting review copies of books:
http://www.lindaformichelli.com/reviewcopy/

When requesting review copies, this site has some very good tips on how to approach a publicist:
http://yodiwan.wordpress.com/2008/08/12/requesting-review-copies-of-books/

For some recent articles on book reviewing and the decline of the standalone book review:
http://www.nytimes.com/2007/05/02/books/02revi.html
http://www.latimes.com/news/opinion/la-op-connelly29apr29,0,3550610.story?coll=la-opinion-rightrail
http://www.onthemedia.org/transcripts/2008/05/23/05
http://www.cjr.org/cover_story/goodbye_to_all_that_1.php?page=all
http://www.insidehighered.com/views/2008/07/23/mclemee
http://bookcriticscircle.blogspot.com/search/label/Book%20Reviewing

Ongoing Blogs about the subject of Book Reviewing. The one on 8/27/08 about reviewing in Alt Weeklies is also good regarding starting to be a book reviewer:
http://bookcriticscircle.blogspot.com/search/label/Book%20Reviewing