It continues to surprise me that Principia Mathematica (PM) still gets mentioned so regularly in discussions related to computer science topics. As far as I can tell, PM was one of the least influential works in any branch of mathematics or logic or philosophy or computer science. It is must be one of the least-read books (3 volumes, 1994 pages) ever written. PM's sole claim to fame is that it was mentioned in Kurt Goedel's famous paper "Ueber formal unentscheidbare Saetze der Principia Mathematica und verwandter Systeme I". The emphasis strongly is on "verwandter Systeme" - related systems, of which PM is just one example.