Braess :
Finite Elemente (German)
Knabner:
Numerical Methods for Elliptic and Parabolic PDEs
Evans, Blackledge, Yardley:
Numerical methods for PDEs
Carsten Carstensen:
Some remarks on the history and future of averaging techniques in a posteriori finite element error analysis
A. Ern and Jean-Luc Guermond:
Theory and Practice of FEs
Examples of Matlab implementation of FEM
David Luenberger:
Optimization by vector space methods