Here are some books (both mathematics related and Christianity related) that I found helpful:

Math

These books were very accessible and taught certain principles fundamental to doing math and thinking about problems.

• How to Read and Do Proofs, by Daniel Solow
• Real Mathematical Analysis, by Charles Pugh
• Calculus: Early Transcendentals, by Jim Stewart
• Applied Partial Differential Equations, by Richard Haberman
• Statistics, by Roger Purves, et al.
• Mathematical Methods in the Physical Sciences, by Mary Boas
• Nonlinear Dynamics and Chaos, by Steven Strogatz
• Finite Volume Methods for Hyperbolic Problems, by Randall Leveque

Christianity

These books profoundly helped me stir my faith and/or laid the foundations of the historic faith.

• The Bible
• Pilgrim’s Progress, by John Bunyan
• Fellowship of the Burning Heart, by A.W. Tozer
• 100 Prison Meditations, Richard Wurmbrand
• Firm Foundations: Creation to Christ, by Trevor McIlwain
• The Gospel’s Power and Message, by Paul Washer
• A Theology for the Church, by Daniel Akin
• Hell’s Best Kept Secret, by Ray Comfort