The question depends so much in your field of interest that it is impossible to give an exhausting answer. Some of my favorites in various fields are the classics

- Nicolas Bourbaki, *General Topology*
- Glen E. Bredon, *Topology and Geometry*
- Shui-Nee Chow and Jack K. Hale, *Methods of Bifurcation Theory*
- Klaus Deimling, *Nonlinear Analysis*
- Albrecht Dold, *Lectures on Algebraic Topology*
- Herbert Federer, *Geometric Measure Theory*
- Morris W. Hirsch, *Differential Topology*
- Thomas J. Jech, *Set Theory*
- Tosio Kato, *Perturbation Theory for Linear Operators*
- Mark A. Krasnoselskij and Petr P. Zabrejko, *Geometrical Methods of Nonlinear Analysis*
- Joram Lindenstrauss and Lior Tzafriri, *Classical Banach Spaces* (2 volumes)
- Jacques-Louis Lions and Enrico Magenes, *Non-Homogeneous Boundary Value Problems and Applications*
- George W. Whitehead, *Elements of Homotopy Theory*
- Eberhard Zeidler, *Nonlinear Functional Analysis and Applications* (several volumes)

Then there are of course a lot of the Lecture Notes and also some very good books in German.