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.