If your interest is mostly academic/philosophic/intellectual, then as bbarr mentioned D.T. Suzuki or Alan Watts are probably the best. They both wrote their works several decades ago but have never really been surpassed as expositors of Buddhism for the Western mind.
Steven Batchelor's works are good but technical and scholarly.
In terms of exploring the "spirit" of Zen Buddhism from an experiential perspective, as Vistesd mentioned Shunryu Suzuki's "Zen Mind, Beginniner's Mind" is excellent. ("Beginner's" in this case is not a reference to a "Zen nubie", but rather to the idea that the true "Zen mind" is always a "beginner mind" in the sense remaining unentangled by time and the dead weight of memory).
My personal all-time favourite Buddhist book is Roshi Kapleau's "The Three Pillars of Zen". This book is simply superb and deeply inspiring for anyone who is at all serious about the whole idea of enlightenment and liberation from suffering. Kapleau was an America who shortly after WWII heard a lecture by D.T. Suzuki which inspired him to travel to Japan. Once there, he spent about 12 years or so in Zen monasteries. After years of what felt like hopeless struggle to crack the mystery of the Zen "koan" (the key to enlightenment), he eventually had his breakthrough and became a Zen master himself. He returned to America and founded a number of Zen centers; he recently died near 100 years of age.
For Tibetan Buddhism, the classic work is "The Tibetan Book of the Dead"; there are several translations. Anything by Chogyam Trungpa is good, or Pema Chodron (who I personally once studied under in the 80s). Also good is Sogyal Rinpoche.
For Theravadin Buddhism, I recommend anything by Joseph Goldstein or Jack Kornfield.
Also good are writings by the old Japanese and Chinese masters, from Hakuin to Huang Po to Bodhidharma, etc.