What are some good books (or other resources) on numerical methods of solving PDEs in 3 space and 1 time variable? I am interested both in finite element and finite volume methods. I could be interested in other methods but I don't know about them. I am interested in being able to take advantage of parallel computing. I have budget to buy some books. I have done quite a bit of 1-space 1-time numerical work. The specific equation I am most interested in is the diffusion equation with quite a few complications. It's transport of neutrons in a nuclear reactor. For example, there are many species diffusing (each is an energy group, as many as 39 in the full model), and these can change from one to the other. The system is changing over time as well as the concentration of species. There are many interesting time scales in the problem from potentially microseconds (if things get badly wrong) up to weeks to account for consumption of fuel. The boundary conditions are interesting. One book already on my to-buy list is Hamming "Numerical Methods for Scientists and Engineers." Any other suggestions?