I believe for the first time ever, the 1981 Cook/Marsh D&D Expert set is now for sale as a PDF at dndclassics.com! I already have the physical boxed set, but am very excited to get it as a PDF too.
What I'm especially looking forward to is making a compiled version of
the Basic and Expert rules in one document, with the pages inter-mixed.
This was what was suggested in the introduction to the Expert set, but I
(and I'm sure many or most others) never had the heart to take a pair
of scissors to our beloved D&D books! It'll be interesting to see
how well it works as a combined book. All going well, I'll be printing
out my own B/X player's book and DM's book :)