Further Reading & References:
This effectively unified Euclidean and non-Euclidean geometries, proving they were not contradictions but different branches of the same mathematical tree. 3. The Great Synthesis Felix Klein | History | Research Starters - EBSCO development of mathematics in the 19th century klein pdf
To guide your reading once you secure a PDF, here are crucial sections and their typical content (based on the Springer reprint edition, which follows the original pagination): development of mathematics in the 19th century klein pdf
The English translation, published by in 1979 (translated by M. Ackerman), is not freely available in PDF form due to copyright. However: development of mathematics in the 19th century klein pdf