He may know his stuff but his stuff does not match up very well with reality. I've run up against him a number of times in the past few months and I've come to realize that he's not very bright and his knowledge of the JFK assassination does not go very deep. He can carry on about the case but so much of what he says and writes is just not true. He now has me blocked.
I've read two of his books.
I found most of his defenses of Jim Garrison unpersuasive but outside of that, I don't think it's inaccurate to say Jim has an encyclopedic knowledge of the JFK assassination investigations.