If you examine the page https://www.isc2.org/Training/Self-Study-Resources then it shows the ISSAP and ISSMP concentrations having an official CBK book, but not the ISSEP. Perhaps this is one to send to ISC2 directly and ask if there's an up to date CBK.
I am preparing ISSEP too.
I don't think there is a "current" version and there is not a single CBK to cover all domain in ISSEP.
you might look at this post
I would say basically you can look at the
NIST 800-160 for enginerering related
where risk related: 800-30 conductingfor risk assessment and 800-37 for RMF
For ISSEP self-study look to the suggested reference list: