Using Moodle book
Using Moodle - Teaching with the Popular Open Source Course Management System by Jason Cole and Helen Foster is published by O'Reilly as part of the Community Press series.
Open publication - Free publishing - More book
The second edition of Using Moodle is freely available online to enjoy, share and modify under a Creative Commons Attribution-NonCommercial-ShareAlike 2.0 License.
See Talk:Using Moodle book for download links for the first edition.