I recently discovered the amazing website Illustration Chronicles: which explores a history of illustration through the images, illustrators and events of the past 175 years. Periodically the editors select a topic to explore. These concepts, such as music, satire, war, and animals, inspire the examples of illustrations that get selected. The project aims to champion the medium and bring some inspiration, insight and knowledge to readers everywhere.
Even if you have just a passing interest in illustration, I think that you will find that Illustration Chronicles is worth a visit. I intend to bookmark the site and return on a regular basis.