overgeeked
B/X Known World
The physical books are great to have. But be careful about trying to run a game straight from the books. The errata on 4E was extensive. Some of it quite necessary to run the game. DCs and damage being off. The early monster books using the wrong math (MM3 and MV are where the "right" math starts). Bursts and blasts being switched around. Different ranges used. Etc. The 4E DMGs are some of the best that's ever been printed for D&D. The available PDFs of the big three have the errata included, if I recall.