I added a header into the main topic list for that. But it is going to be later in the Advanced Topics, section. Oddly, I know of pretty much no standard Linux Administration tomes that cover DRBD. It's so core, very odd that it so often gets missed.
Could it be that most Linux Admin's don't know about it until they go searching for it? ...That's how I found out about it.
You would hope that the people writing the books would know, though!