Curry's PhD dissertation  Introduction to Combinatory Logic, represents a first major work on a new subject.  Curry's interest in the subject started when he noticed the complicated form of substitution in Principia Mathematica and set about trying   to find a simpler form  of this rule. This led him by 1926 to some of the combinators. In 1928 Curry went to Gottingen and completed his dissertation in 1929 under the direction of David Hilbert.  This book is a translation of the dissertation. 

 Curry's dissertation was...