It's not a book but Michael Wiesenberg wrote a series of very good articles on online 5cd.
Here's a link to the articles:
http://www.readybetgo.com/poker/online-draw/
Also Ryan Fizler made a bunch of 5cd training videos and strategy articles at Real Poker Training that are really good and helped me alot.
Here's a link to that:
http://www.realpokertraining.com/
Hope this helped