DeeperBlue would probably recommend the following:
We've gathered the top 10 essential books that any freediver needs
www.deeperblue.com
The only reading I've done on the subject has been from freediving classes (i.e. FII Levels I and II). If freediving is something you're looking to get into, I'd recommend looking around for a comparable course to take. It will help with safety, good practices, and knowledge.
How comfortable are you feeling in the water? If you're in the open water, make sure you have a warm suit on and a partner. Temperature comfort makes a huge difference in your ability to hold your breath. Good luck out there.