The Finite Element Method, its basis & fundamentals, O.C. Zienkiewicz, R.L. Taylor, J.Z. Zhu, 6th edition ISBN 0-7506-6320-0.
Again the FEM study leads to unexpected interesting stuff, such as Sage, a wonderful Math package. If you want to explore Math and you are not (yet) prepared to buy Maple or Mathematica, start with Sage.
It also helps with your ‘homework’ for The Theoretical Minimum, “what you need to know to start doing physics” by Leonard Susskind and George Hrabovsky.