This next article would have to be the best ever about the sales difference between marketing a book and building a platform. You have to get near the end before you come across Scenario 1 (with all the common mistakes new authors make in trying to sell their first book) and Scenario 2 (a different approach with regular blogging, and an emphasis on being helpful), but they are 'Must Read': http://timgrahl.com/when-should-you-start-marketing-your-book/?utm_content=buffer36c1d&utm_medium=social&utm_source=twitter.com&utm_campaign=buffer . The top take home tip is 'Your number 1 job as a writer is not to simply write your next book, but to build a platform that will support your entire career.'
Of course, illustrators should also be building a platform too!