|Screen shot from the bonus catalog at the end of the new Expert Set rulebook pdf|
As I wrote on the Rules Expansion page on the ZA site:
In Dragon #35, Gygax also revealed that "Design is now hard at work on the second boxed D&D game, the Expert Set. It will take players through at least 12th level of experience, tie in the best of the “Original” material, and actually add some new classes, spells, magic, monsters and so on." In context, this appeared to be an Expert Set that would complement the Holmes Basic Set. A late 1980 Gateway to Adventure catalog shows the Holmes Basic set side-by-side with an unpictured but soon-to-be-released Expert Set. However, when the Expert Set finally arrived in 1981 it was accompanied by an entirely new Basic Set that replaced the Holmes Basic set (although TSR continued to have the original in stock through the Mail Order Hobby Shop until at least 1986). To aid owners of the original Basic set who did not wish to buy a new set, the Cook/Marsh rulebook contained a section on page X4 titled "Using D&D Expert with an early edition of D&D Basic" which began "If your copy of the D&D Basic rules has a blue cover with a picture of a dragon on it, then this section is for you". This section then provides a summary of the "new material found in the 2nd edition of D&D Basic.
See also the previous post, Holmes Expert Set from TSR catalog.
Update: I ended up buying the new pdf of the Expert Set rulebook. See a future post for some notes.