Jean-Pierre Serre had a lot of original theorems (some due to him and some due to John Tate) published originally in textbooks. To give two examples, his book "Cohomologie Galoisienne" ("Galois cohomology") contain, I believe, the first published proof of many theorems of Tate that form the socle of the theory. His book "abelian $l$-adic representations and elliptic curves" contains the proof of his theorem that two elliptic curves over a number field with non-integral j-invariants that have the same $L$-functions are isogenous.