If you want to prepare for the CCRN I recommend CCRN by Kaplan. Divided into systems the book is
to the point without a bunch of destracting details that aren't necessary. Would also get the book from the AACN website, this has practice tests with rationales. I used those two books to prepare and passed the test no problem.
Just for a nice reference I recommend "Notes on ICU Nursing" by Mark Hammerschmidt. Good info for newbies and longtimers as well.