If you're interested in Zen, I heartily recommend the following books, more or less in the order shown.
Basic requirements
Dropping Ashes on the Buddha, by Zen Master Sung Sahn (ZMSS). This is generally a collection of letters between ZMSS and his students, essentially a teaching of Zen by postal mail. A great introduction.
Zen Training, by Katsuki Sekida. Discusses the physical practice of Zen, including how to sit, breathe, and maintain your posture.
Still interested?
If you don't have much money, those first two books will take you a long way in your understanding. Otherwise, these other books are also wonderful:
Any book by ZMSS is a terrific read. It is also helpful to follow along with one Zen Master, as bouncing from one to another may only confuse you more.
I love two books by Shunryu Suzuki: "Zen Mind, Beginner's Mind", and "not always so." I would like to have met this kind Zen Master.
"An Introduction to Zen Buddhism" by D.T. Suzuki was the second Zen book I ever purchased, but it can be a hard read, so I recommend it for later.
"Questions to a Zen Master" by Taizen Deshimaru is very different, and I recommend it because of this difference.
"Zen Master Raven" is a cute, cartoon-like book by Robert Aiken. With its simple one- or two-page stories and cartoons, it can be good for children, but I also like to review it from time to time.
Speaking of cartoons, "Zen Speaks: Shouts of Nothingness" by Tsai Chih Chung is a tremendous book about Zen and Zen History, in the form of an ongoing cartoon strip.
"The Gateless Barrier, Zen Comments on the Mumonkan" by Zenkei Shibayama is helpful in explaining some things about Zen koans. If I remember right, it is this book that mentions that in old times, the Chinese would take their sandals off and place them on their heads when walking in a funeral ceremony. It is very helpful to know this history when you later read a koan where a Zen Master takes his sandals off and places them on his head.
Even more
If/when you're ready for more, there are no dozens, possibly hundreds of Zen books. Here are some more favorites:
"After the Ecstasy, the Laundry" by Jack Kornfield certainly has my favorite title.
All of Thich Nhat Hanh's books can be very good, especially for the right person. For me, his writing is very gentle and peaceful, while my body or soul seems to come with a certain amount of angst. :)
"The Recorded Sayings of Zen Master Joshu" can be very interesting. Again, it's nice to follow one Zen Master, and not bounce between multiple ones.
"The Zen Teaching of Bodhidharma", translated by Red Pine, is also on the top of my bookshelf. It has several nice lines that hit the spot.
"The Tibetan Yogas of Dream and Sleep" by Tenzin Wangyal Rinpoche has also been a good read of late. Because of my interest in dreaming and lucid dreaming, this one also hits the spot for me.
The Dalai Lama
You may think I am remiss for not mentioning all of the Dalai Lama's books yet, and yes, I do own many of them. But IMHO he writes more about Buddhist philosophy, as opposed to my interest in Zen and koans. If you are interested in Buddhism, then yes, he is absolutely the place to start. If you are interested in Zen, zazen, and koans, then my suggestions above should work.