Just provisionally pass the CAP exam.
Unlike CISSP and others, I did not specifically prepare for this exam. I find my knowledge when I study for CISSP, CSSLP, ISSAP and ISSEP should be good enough for the exam.
Just look at the question and answer at the CAP CBK, glance through the flashcard, revisit some NIST SP and got passed.
Congratulations. Maybe you had a version that was compatible with the knowledge you already had.
The CAP CBK is outdated, but you can still use this book for fundamental concepts. For the upcoming new version of the CAP exam, I'd still use the 2nd edition CBK, but I'd also check out NIST's latest pubs such as SP 800-53 Rev 5, 800-37 Rev 2, 800-137A, etc. Basically, the most fundamental NIST pubs used for references are listed at the end of each subtask of each of the RMF steps in NIST SP 800-37 Rev 2. I'd definitely glance at each of these pubs for the upcoming version of the CAP exam.