I strongly believe that Showcase Nos. 9 and 10 need to be listed in the canonical sources as they are solo Lois Lane comics that directly paved the way for her own series, which is canonical. Additionally, I believe that the Earth-2 Power Girl issues of Showcase, No. 97-99, should also be included.
And while I know that this series is not well-liked by some (most?) of us, I also believe that Steve Gerber's Phantom Zone miniseries should be included as well, since it was published by DC prior to 1986 and featured the Man of Steel and his supporting cast.
Those Showcase issues would make great additions. IMHO. Perhaps we should make a poll and vote on it after this thread runs its course.
Phantom Zone miniseries was voted against before because it show the Phantom Zone so grossly out of character, there was a lot of stuff added to it that came out of nowhere and perhaps never show up again that I know of, I don't know if it was ever reference in any of the canon books. If someone can prove that it was referenced, then perhaps we should rethink adding it or not.