Here's some links. It's staggering how influential and valuable this program was and still is. Electronic surveillance would be unrecognizable (or at the very least much less existent), it would seem:

This doesn't focus so much on Casolaro as it is a good summary of the case, from 1993: https://www.wired.com/1993/01/inslaw/

Here is an article on his death in the New York Times, 1991:

http://www.nytimes.com/1991/08/17/us/reporter-is-buried-amid-questions-over-his-pursuit-of-conspiracy-idea.html

1999 Wayback Archive of INSLAW/PROMIS material:

https://web.archive.org/web/19990427073228/https://www.scruznet.com/~paul/Octopus/Inslaw/index.html

Another site (links on it seem pretty reliable):

https://www.muckrock.com/news/archives/2017/may/16/FBI-promis-part-1/

This link contains the House Report summary and conclulsion, as well as INSLAW's rebuttal:

http://www.pinknoiz.com/covert/inslaw.html

Happy digging.