After a little brainstorming session I decided to hit up the local thrift store to swipe their Dr. Seuss selection. I went for the oldest and most authentic books I could find, meaning, books that were written by Dr. Seuss himself and with the pages actually sewn into the spine (none of that glued in stuff).
Once home, I quickly went to work coming up with some Dr. Seuss inspired copy for the invitation itself (click on pic above to zoom in). Then I promptly went to work disassembling the books I had purchases earlier.
UPDATE: I now have this package for purchase in digital form. The package includes files of invitation, envelope template and step-by-step instructions on how to create your own handmade Dr. Seuss birthday invitations as shown above. To purchase check out the shop HERE. Please email me if you have any questions (gnat[dot]anderson[at]gmail.com).