Sometimes my agent recieves e-mails from different publishing houses. In turn I will recieve a manuscript and be asked to do a character sketch for a story. It can be a lot of work and there's no guarantee that they will choose you to illustrate their book.
But one nice aspect of doing a wide variety of illustrations is you learn new things by doing research into subjects that you may never pay attention to in day to day life. Today I was researching the La Raspa (a traditional Mexican dance).
This is a sketch that I will soon add color to.