... are there books, or possibly online resources or even brain exercises (?)
Well, a college CS course can be a brain exercise -- at least that's what's usually intended.
I don't know your location, but do you have access to a good university or junior college nearby? You may want to see if they offer a good CS curriculum where you could possibly audit courses. University extension courses are usually strictly "technology" (one specific language or one specific technique), so they may not offer the more broad training you're looking for, but those are a possibility as well.
My main point is that if you think it could broaden your view to take a "fundamentals" course, avail yourself of it. Don't worry about "It's too late", or "I'm self-taught and proud of it", or labels like that. Money is of course a consideration, but you may be able to finagle an audit-only situation.