As someone who has done reviews in the past (on music and movies not books, though), here's my take on the matter.  If I like the thing well enough then I am not just going to write a review about but I am going to recommend it personally to friends outside of any review I might post.  Furthermore, if it's by an author with more than one work to their name then I am going to track down other works by that author.   If I don't like it then I would still probably give the book or CD away to a friend in hopes that they might like it more than me.  I've given a few away simply to introduce someone to an artist or author who I think they would benefit from being familiar with the works of.