For quantum Monte Carlo or exact/Lanczos diagonalization
I recommend Computational Studies of Quantum Spin Systems by Anders Sandvik arXiv:1101.3281, or AIP Conf. Pro. 1297, 135 (2010).
This review is designed to teach you how to write quantum Monte Carlo simulations of spin systems like the Heisenberg model. There are also example codes from the workshop on Sandvik's website (note of caution: there may be a bug where the random number generator doesn't work for the SSE code, so you may have to substitute your own RNG).