Since the content of Tolle's books has been covered well elsewhere, I'd like to address the person new to Tolle and wondering where to begin.
In my personal experience, if I could only afford to purchase one book, I'd start with PRACTICING THE POWER OF NOW. I feel that it essentially restates the material found in the original POWER OF NOW but in a format I found easier to digest. I don't see much point in owning both books (which I currently do.)
If you connect with PRACTICING and want more, then I'd recommend the audio book/retreat Realizing the Power of Now: An In-Depth Retreat With Eckhart Tolle. (It's less than $10 if you purchase the audio file from Audible.) I'm only half way through listening yet I constantly find myself thinking "Ah...now I understand what he meant." It truly is an excellent supplement.
There is one more Tolle book here, STILLNESS SPEAKS. I borrowed this from the library. It's basically just out-takes from the other books, sort of a quotable quotes book that restates things in a nutshell. It would make a nice gift to a Tolle fan, but again, doesn't offer anything more than the other books. I suppose my message is: everything you need to know is in the original POWER OF NOW, stated perhaps more clearly in PRACTICING and excerpted in STILLNESS but supported well in audio format by the retreat CD.
I find Tolle's basic message very powerful, enlightening and simple. As he himself constantly repeats, he's not saying anything new...be here now. But he explains it in a way which finally connects the dots for me. Unfortunately, this simple message seems to be getting constantly repackaged and I hope it doesn't turn into a "Chicken Soup for the Enlightened Soul" series. (I should probably go make sure that's not the title of an actual book! <g>)
Curator, AfroAmericanHeritage dot com