Louis Comtet, through his book "Analyse Combinatoire vol 1 and 2", now republished in english translation with additions and corrections as "Advanced Combinatorics".
When ? My first year in Paris University while I was attending boring courses in Analysis and Linear Algebra that were very inferior to what I have been exposed in high school the year before.
These two little pocket books were relatively easy and cheap to find and gave a wealth of packed information and links to the existing litterature on combinatorics. Combinatorial Mathematics were not in fashion in France in the 1970s, neither in the 1980s. Among many things I liked were the fancy notations, the diagrams, the density of results, the careful index, the intersection with so many other mathematical theories such as set theory, differential equations, topology, group theory. And it was also my first contact with a slightly formalized graph theory, Eulerian numbers, integer partitions, multiple summation, etc.