I'd (also) buy that!Ultimate Toolbox, Vol 2.
A map-making program that allows one to easily design three-dimensional spaces, so that it is easy to see how various levels and sublevels link up in a complex environment. The program I am using now is called "markers and graph paper".
RC
One more thing I'd buy on spec: 4e Modern.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.