I know we're not allowed to post story ideas but reference books are not stories and I think I have a good idea for a reference book.
Also, setting aside any possible legal concerns, this isn't really a place where pitching it would go anywhere anyway; if you want to make it yourself, you should pitch it to publishers, and if it's "someone should write this" then nothing would probably come of it even if you did post it. Authors usually have no shortage of ideas already, and (no offense to you) they'd probably rather use their own ideas than someone else's.
I agree that any ideas for a reference book are unlikely to amount to anything, but what's wrong with saying "Wouldn't it be cool to have a reference book on x?" That's pretty generic. If you were actually posting an outline of the book, then that would clearly be objectionable. But why can't you say something like "Wouldn't it be cool to have a Star Trek version of Star Wars's Essential Guides on the various sorts of topics they covered"?
If that's all he wants to do, then yeah sure, people do that all the time on this board. It just sounded to me from that first post more like it's something he was hoping someone would end up picking up and running with.
It's the level of detail that makes a difference - if it's a general idea of a subject you'd like to see covered then it'll be fine, but if you go into detail of what would be covered and how you would like it to deal with things then that's crossing the line.