We want to make it easy for you to get the books that you want. To do this purely through pledge levels we'd have to offer a confusing myriad of additional pledge levels. Instead of that we're offering you the option to upgrade your books.
If you've chosen one of the higher pledge levels (such as Diabolical Dark Young) and wish to receive a more expensive level of book than the one stated on that pledge level you may do so by increasing your pledged amount and letting us know exactly what you want.
We'll list the possible options along with the extra cost below.
1. DIABOLICAL DARK YOUNG UPGRADE: you automatically receive everything on the SLOBBERING SHOGGOTH pledge level. If you wish to receive the leatherette Book set instead (as offered for the pledge level NICTITATING NYARLATHOTEP) add $233 to your pledge (that's simply the difference in cost between SLOBBERING SHOGGOTH and NICTITATING NYARLATHOTEP). You would then get the game at Gencon plus one leatherette deluxe Investigator Handbook and one leatherette Keeper's Rulebook (along with the other extras).
If you want a combination of pledge level and books that we haven't listed, please tell us and we'll list it below.