There are many resources to learn about DFT. My recommendation is currently as follows:
.1If you are modeling materials, as is standard in our group:
.aRead at minimum Chapters 1-3 of Density Functional Theory: A Practical Introduction by Sholl and Steckel. It is all relevant though.
.2If you are modeling molecules rather than materials:
.aRead Chapter 8 and 10 of Essentials of Computational Chemistry: Theories and Models.
.bAs-needed, refer to Introduction to Computational Chemistry by Jensen for select topics of interest.