I have that one as well. A couple of other neat ones I have in my library ( may be out of print ), both from Dover, are:
"Geometry, Relativity and the Fourth Dimension " by Rudolf v. B. Rucker
"Taxicab Geometry, An Adventure in Non-Euclidean Geometry " by Eugene F. Krause
Neat little books that sort of take off from where Flatland left off...
Rudy Rucker has also written a number of science fiction stories, and they're *very* mindbending -- like taking a drug trip via text. Even calling them "science fiction" doesn't really capture their weirdness. Perhaps they're better described as "math fiction" or "physics fiction"...