Books about synthetic projective geometry Are there books in English about synthetic projective geometry?  More specifically, results of Karl von Staudt  (imaginary elements theory through elliptical involutions, imaginary circle, infinity's imaginary circle)

There's lots to learn from "modern" texts such as Richter-Gebert, Coxeter, et al.  But the heyday of synthetic projective geometry appears to have been in the 19th and early 20th centuries.  After that, both research and pedagogy moved to other topics in math.
For the older texts archive.org is your friend, going back to Poncelet's groundbreaking Traité des propriétés projectives des figures which introduces the principle of continuity, an early view of imaginary elements.
Some examples: Milne's Cross-Ratio Geometry (which makes reference to imaginary elements), and Pickford's Elementary Projective Geometry.
Dialing in on your interest in imaginary elements, Chapter XXVII of Russel's Pure Geometry discusses imaginary points and lines and gives some practical constructions.
Also check out Coolidge's Geometry of the Complex Domain (the final chapter is on Von Staudt Theory) and Hatton's Theory of the Imaginary in Geometry.
