First off, I want to say (for copyright reasons) that all the things in the picture are from Grossman's book "The Magicians." Just so no one comes breathing down my neck about his clock trees.
"The Magicians" is not a kid's novel – it would be, as it's a mix of Narnia- and Harry Potter-type fiction, except that it's also got "Less Than Zero" thrown in there as well. So the main characters are all party-hard, big-city-jaded slackers who happen to go to magic school and (more improbably) want to find a magic land of their childhood. It's a readable book, even if the three elements don't always mesh well together.
The best part for me, though, was the "Fillory and Further" series (his faux Narnia books) that his characters are supposed to have read. They sound really entertaining, which makes me wish he'd actually written those. He even has a fake first chapter of "The World in the Walls" on one of the book's websites.
In addition, there is a fake fan site for the fake books, which includes fake fan art. That's what I really wanted to draw this for – I sent it to them to see if I could have it put up as a fake fan. Alas, as of posting it has gone unregarded.
Since one of his inspirations was Narnia, I chose Pauline Baynes (the original Narnia illustrator) as my inspiration for the drawing. She's got some amazing stuff – I like her work. If nothing else, trying to do this gave me an excuse to look up her work just to check out the style.