I really enjoy Rudy Rucker's nonfiction, and some of his fiction too (_White Light_ is great). He's very good at presenting mind-blowingly cool ideas in accessible expository prose, and he knows _just_ when to throw in the bombs.
This particular book is published by Dover, and it's not one of their usual reprints; it was _originally_ published by Dover. (In 1977, but the geometry of spacetime hasn't changed much since then.) It's an exploration of just what the title says: the geometry of the four-dimensional spacetime that the theory of relativity says is Really Out There.
Well, this is a good book on the subject, but you can get others (although one of the best -- Cornelius Lanczos's delightful _Space Through the Ages_ -- has long been out of print). What's coolest about this one is that Rudy Rucker wrote it.
Which means you get those little bombs thrown in at all the right places. Of course Rucker gives you what any competent mathematician will give you -- a sound introductory presentation of the mathematics of 4D spacetime and relativity theory, which are weird enough if you haven't encountered them before (and maybe even if you have) -- but he doesn't stop there. You also get an argument that the apparent passage of time is an illusion, and a little speculation about how this might tie in with the Many-Worlds Interpretation of quantum mechanics. And even that isn't all: you get a suggestion that it's possible to _develop a spacetime consciousness_ via some sort of meditation techniques or mystical insight, together with an entry in the annotated bibliography referring you (cautiously) to Robert A. Monroe's _Journeys Out of the Body_, whose experiments Rucker himself has tried.
It's like Raymond Smullyan on acid, if you know what I mean. But honest, it really does make sense. And it really will knock your mind loose from your brain even without the use of chemical aids.
This is the sort of thing Rucker does best. He does it in _Infinity and the Mind_, too (with which this volume has a little bit of overlap, but you won't care). Check out that book as well, along with _White Light_. Mathematical hippie mysticism just doesn't get any better.