Many of the readers are good with a few as good as professional book narrators. As with any organization there are a handful of readers that really stink.
My district/county library with MeL (Michigan eLibrary http://elibrary.mel.org/se
Free but donations requested: Podiobooks.com (http://podiobooks.com) is a site were authors can self publish the books as audio books. Many of the books are read by the author. Some authors have family members or friends do the reading. Podiobooks RSS feeds allow you to get an episode/chapter a day, week, month, every other day, as authors post episodes/chapters or if the book is completed, all episodes at once.
Although you can download books for free, they do ask for a donation that is split between Podiobooks and the author.
One problem with books from Librivox and Podiobooks, is being that readers/recorders are inexperienced may of the files are record at well below normal volume levels. I use audio editing software to increase volume levels to normal so I don't have to crank the volume all the way up on my player to hear the book.
Purchased books:
I buy download versions of audio books from Graphic Audio (http://graphicaudio.net) and Audible (http://audible.com).
Graphic audio works with authors of action serial books to create dramatize with sound effects audio books. Genres include westerns, fighting terrorists, heroes with super powers, deep space/future world, post nuclear holocaust, fantasy and DC Comics.
All books are $12.99 for MP3 downloads/MP3-CD, $19.99 for audio CD's, box sets of 5 to 10 starting at $49.99.
I also have an Audible membership an get 2 books a month for $22.95 in addition to any other books I purchase. I use Audible to get audio books that aren't available through the library.
