Most people forget things they're not using. Since you couldn't come up with anything at all and it's just this one book, and not up to date psychology, it sounds like you're not that interested and maybe were expecting something different, though.

I would have no problem recalling many of the main details from having studied psychology, but then it was more concrete, studies, diagnostic criteria applicable to real people. You may not be able to remember because it was a psychotherapy book and wasn't really saying anything, just waffle trying to sound meaningful, that stuff is bunk, so it's not surprising if there was nothing for your memory to keep hold of.