Book Birthday by French Fries & Waffles --this is more of a comic (be sure to go read it up close!), but I still think it'd be cute as a poster. And that sounds like the perfect birthday to me! You can buy some of the artist's prints here.
I don't know if this is available to print anywhere, but how cool is this vintage summer reading poster by Rudi Bass? You can see more of them on Stickers & Stuff's blog.
I love Maurice Sendak so much and this vacation poster is adorable. He did a whole book of reading posters that you can view on BrainPickings.org and buy on Amazon. Of course, the wild things are my favorite, but I love this sea dragon too.
And finally, this neat book print by Fifth & Hazel. I love the basic drawings of the books and searching to see if you're favorite title is on there. You can buy and print your own black & white version here.