I am a Theravada Buddhist whose practice is centered on Ānāpānasati. Most of what I read is Theravada oriented, such as the Pali Canon, etc.; with one exception: B. Alan Wallace (Buddhist with a strong Tibetan orientation).
I have read several of his books and I greatly enjoy his scholarship and very great scope of knowledge—including insights about the Theravada tradition as well. I just finished The Four Immeasurables and I will gladly share that it is my favorite of B. Alan Wallace books read so far.
An interesting point about this books is that less than half of the book is dedicated to the four immeasurables, Shamatha is given more than equal billing along with some very interesting questions and answers where I feel Wallace shines.
The Shamatha portion that opens the book (the first 80+ pages) is a beautiful overview of the Tibetan approach to concentration leading all the way up to, but not including, the first Jhāna. The next 80 pages concern loving-kindness, compassion, sympathetic joy, and equanimity, i.e., the four; and he treats them beautifully. He then rounds the book out with a very trenchant chapter on the Empowerment of Insight, which on its own is worth the fee of admission.
Buy this book. Read it. Cherish it. You will not regret it.