Ten years ago, I wrote about the newly released PDF of the 1981 D&D Expert Rules at DriveThruRPG. Now, they've finally added a Print-On-Demand (POD) option for both the Expert Rules and the 1981 verison of the Isle of Dread, the two main components of the 1981 Expert Set (other than dice & crayon). Find them here at DriveThruRPG:
It's not widely known, but production on these rules started out as an expansion to the Holmes Basic rules. In March 1980, in Dragon #35, Gygax 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 instead accompanied by an entirely new Basic Set, which replaced the Holmes Basic set (although TSR continued to sell old stock of the rulebook through the Mail Order Hobby Shop until at least 1986). To aid owners of the original Basic rules who did not wish to buy the new Basic rulebook or 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".
Over on the Holmes Ref page, I have a reference sheet with character info bridging Holmes with Cook/Marsh up to sixth level, titled "Holmes Basic / Cook Expert Reference Sheet v1.0".