Section 6.7 Reference Topics
A super interactive site exploring some of the ideas of nonEuclidean geometry is found using
this link:
http://math.rice.edu/~joel/NonEuclid/
This site explores Saccheri's quadrilateral:
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Non-Euclidean_geometry.html