I'd heard about Heinrich Dumoulin's two volume History of Zen a number of times, but Dumoulin not being a Buddhist himself led me to avoid it. However, I ran into the set at a library recently, and to my surprise it's actually reading pretty well, and appears to be somewhat objective / extensive.
What I'm curious about is the good and bad of the book. What did he get right, and what did he get wrong about Zen's history? Essentially a short critical analysis of this book and where / where I shouldn't be skeptical.
If there are too many factual errors in the book to list them all, then calling this out and a few small examples should suffice.