Well, since you already have an ATP you won't have to take the written (at least, if it's a USA ATP), so I assume you're just trying to get up to speed for the oral part of the practical test. In that case, the FAA's old textbook, the
Rotorcraft Flying Handbook is worth a read. That book was written in 2000. Previously we used a reprint of the Army Field Training manual which was both sad and funny (for the happy and sad Bell 47 pictures). The Rotorcraft Flying Handbook is divided into two parts: the first part is helicopters, and the second part is gyrocopters, so I suggest you just read the first part.
The current textbook
The Helicopter Flying Handbook is pretty bad. I'm not sure why they thought they needed a rewrite vs just doing a little update, but it's filled with lots of mistakes. While the Rotorcraft Flying Handbook seems to have been authored by someone knowledgable, the Helicopter Flying Handbook appears to have been edited together from 3-4 sources by someone not knowledgable about helicopters. There are some pretty bad mistakes in it.
Those two books are free for the downloading. I also would recommend
Principles of Helicopter Flight by W.J. Wagtendonk, and
Cyclic and Collective by our own Shawn Coyle. There are other good books out there, but those are the three I would recommend (I don't recommend the Helicopter Flying Handbook unless you have to take the FAA written - then you want to read it just to pass the written test).