CSSLP was mentioned next to CISSP and eqvivalents in a job description by an international financial institution who was looking for mid-level manager specialized to web security. Also I have got much more job offers for leading positions after passing CSSLP.
I used for studying next to CSSLP CBK tutorial videos by Skillsoft and some tutorials form Youtube. Answering to example questions was also really helpful for me.
Core Software Security by JAMES RANSOME,ANMOL MISRA is a nice book in addition to the othe books
“The 7 Qualities of Highly Secure Software" is a good supplementary
A Bug Hunter’s Diary by by Tobias Klein is excellent too for gaining software dev sec and testing
CLASP is also very good stuff to refer for a significant step forward compared to traditional ad-hoc approaches to software security.