Father Dumoulin's magisterial two volume history of Zen Buddhism is the standard English work on the subject and come highly recommended for any student of Zen of Eastern religions who wants a scholarly overview of the subject. My only caveat would be that this is NOT a history of Zen thought in the sense of a detailed analysis of the various philosophical doctrines of the various schools of Zen. Such a work would be much, much larger than this one. It does, of course, deal with these issues, but only in a very basic way, as one would expect from a general history. In short, it is the best general history of Zen available.

It's a real shame that this book is out of print. This is one of the great, classic studies of the history of the development of Zen. Volume 1 (this one) covers the antecedents of Zen in India to its development in China. Volume 2 covers Zen in Japan. I've only read volume 1, but I learned a lot about both the history of Zen and also its philosophy. Dumoulin is one of the great scholars of the history of Zen, and although he is not always philosophically acute, you will get some genuine insights into Zen doctrine by reading this. (Of course, according to Zen, the doctrine isn't really what's important!)