This is a list of books and papers available online that I have found useful.

A Geometric Approach to Differential Forms by David Bachman
Course of Differential Geometry by R.A. Sharipov
Div, Grad, Curl are Dead by William Burke
Differential Geometry in Physics by Gabriel Lugo
Elementary Differential Geometry by Bob Jantzen
Path Integral Methods and Applications by Richard MacKenzie
Classical Entanglement by Douglas G. Danforth
Scientific Software Engineering in a Nutshell by Helmut G. Katzgraber
Computational Complexity for Physicists by Stephan Mertens
Introduction to Tensor Calculus and Continuum Mechanics by John Heinbockel
Foundations of Mechanics by Ralph Abraham and Jerrold Marsden
Global Optimization Algorithms: Theory and Application by Thomas Weise
Complex Analysis by George Cain
How to Read a Book by Mortimer Adler and Charles van Doren

Suggestions For Giving Talks by Robert Geroch
The importance of stupidity in scientific research by Martin A. Schwartz