Flatland by E.A. Abbott
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...