Collector's Arcana: The box is in fact a sturdy slipcase. The box contains facsimiles of famous adventure modules from the past, the D&D rulebook, an Easley art print.
There is an interesting booklet about the history of TSR. It is the updated version of the in-house production "20 Years of TSR",
which was released internally with a print run of 300 copies.
There's a foreword by E. Gary Gygax, an introduction into the classic reprints by
Ryan Dancey, and reminiscences from famous game designers
(Mohan, Skip Williams,
Winter, Pickens, et al.) from the past,
who each tell a story from their past with TSR.
The most interesting item to gamers is the adventure module L3 - Deep Dwarven Delve,
which is the conclusion of the L-series.
It was written by Len Lakofka in 1979, but never published.
As part of the silver anniversary celebrations, WOTC released the item as part of the Silver Anniversary boxed set.
Sometimes, sellers split the Anniversary boxes up and sell L3 individually. If you buy a box without shrinkwrap, make sure all the items are there and check
especially for the presence of L3. L3 commands a premium on eBay (up to $40), and the box without it is worth very little.
The facsimiles each bear the TSR Silver Anniversary logo.
Shrinkwrapped copies add 100% to the base value.
(from L3 - Deep Dwarven Delve): " Your party of stalwart heroes must venture into a lost dwarven mining complex, fighting terrible monsters,
bypassing deadly traps, and exploring chambers heavy with the dust of ages. Somewhere deep below the surface lies the heart if darkness—a corrupting evil that must be stopped before its influence can spread!"