I've been doing tangles for about 4 years. You don't need books. Just go to
www.tanglepatterns.com and you'll find hundreds of patterns there that you can download (with instructions on how to draw them), along with links to dozens and dozens of other sites to give you inspiration. Or go to Pinterest and search "tangle patterns". You also don't need the expensive paper and pens that Zentangle sells. Any old paper will do to start, and use a fine-line marker (such as a Sharpie or Micron 03 or smaller). Once I really started getting into it I started buying cardstock from Hobby Lobby in the 4.5x6.5" size. They sell it for $4.99 for 100 sheets, and often have it on sale for half price. Tangling doesn't have to be expensive, nor do you have to do it their way. It's truly just doodling to your heart's content!