@laracissp Unfortunately the ISSEP is kind of the oddball of the former concentrations. It was sort of retrofitted for the NSA. I did read the only CBK for historical purposes knowing that it was not testable. The document that was mostly based on is no longer used by the NSA (ITAF) and did not read it. I took it a couple of years ago before the recent exam change. I used the NIST documents. Some I am very familiar with but read the ones that was not. I do think that having the CAP (now CGRC) was an advantage. I used this for the NIST references. I didn’t use any of the other references besides the NIST ones: https://www.isc2.org/certifications/references#ISSEP. I did read PMI’s Agile practice guide but did not really feel like it helped me. Best wishes.
When it comes to the ISSEP exam, the best starting points for exam preparation are the Exam Outline and the list of references that were used when developing the exam content. You can find both of those here:
