These are some of the non-textbooks which I've enjoyed reading. Maybe there are a few ideas for your own reading list.

• How to Solve It
• Mathematics and Plausible Reasoning
• Pilgrim’s Progress
• Man’s Search for Meaning
• The Lord of the Rings
• The Federalist Papers
• The 7 Habits of Highly Effective People
• Around the World in 80 Days
• The Courage to Be Disliked
• The Upside of Stress
• Medieval Technology and Social Change
• The Road Less Traveled
• The Inner Game of Tennis
• The Sovereign Individual
• Flow
• Mindset
• The Narrow Road to the Deep North and Other Travel Sketches
• The American Technological Challenge
• The Analects
• The Art of Doing Science and Engineering
• The Dip

As for textbooks, I've found the following books useful:

• Foundations of Modern Probability (O. Kallenberg)
• Random Measures: Theory and Applications (O. Kallenberg)
• Probability: Theory and Examples (R. Durrett)
• An Introduction to Measure Theory (T. Tao)
• Analysis I & II (T. Tao)
• Complex Analysis (E. Stein and R. Shakarchi)
• Modern Real Analysis (W.P. Ziemer)
• Algebra (T.W. Hungerford)
• Topology (J. Munkres)
• Set Theory (T. Jech)
• Partial Differential Equations (L.C. Evans)
• Convex Analysis (R.T. Rockafellar)
• Convex Optimization (S. Boyd and L. Vandenberghe)
• The Probabilistic Method (N. Alon and J.H. Spencer)
• Introduction to the Theory of Computation (M. Sipser)
• Elements of Information Theory (T.M. Cover and J.A. Thomas)