Also you can find many resources online such as:

It’s dense, it’s rigorous, and it’s arguably one of the most complete textbooks for mastering the math behind modern engineering. Mathematical Association of America (MAA) Find more details at SIAM Publications

: Guarantees a unique fixed point if the mapping strictly shrinks distances. Brouwer and Schauder Fixed Point Theorems

Linear and nonlinear functional analysis form complementary pillars of modern analysis. Linear theory provides precise structural and spectral tools; nonlinear theory supplies flexible topological and variational methods for complex phenomena. Together they underpin much of contemporary PDE theory, applied mathematics, and mathematical physics.

Ciarlet sits between Brezis (pure linear + nonlinear PDEs) and Zeidler (5 volumes!). It is more applied than Brezis, more linear-focused than Zeidler, and more modern than Yosida.