It's amazing to see that guy still going. I remember seeing a PDF of his first book which was, if I remember correctly, largely a bunch of pasted unix man pages for various security tools and bits and pieces of guidebooks from the linux documentation project.

This was probably 10+ years ago so I don't want to make any judgements about his work now. If he has carried on as he started it's hard to believe he has managed to make a following for himself.

I guess if you publish enough stuff over a long enough time people come to see that as evidence of credibility.