Complex codebooks with small inner-product correlation have many applications such as in code-division multiple access communications and compressed sensing. It is desirable but difficult to construct optimal codebooks achieving the well-known Welch bound. In this paper, complex codebooks are investigated from a graph theoretic perspective. A connection between codebooks and Cayley sum graphs is established. Based on this, many infinite families of complex codebooks are explicitly constructed, which are asymptotically optimal with respect to the Welch bound. These constructions not only include some known constructions as special cases but also provide flexible new parameters.