Midnight Days

Neil Gaiman's Midnight Days (New York, 1999), like Alan Moore's Across The Universe, collects its author's DC Universe short fiction. In Gaiman's case, these comprise:

a never before drawn or published Swamp Thing short story;
a Swamp Thing annual about what Swampy's friends did while he was time traveling in the monthly comic written by Rick Veitch;
a short story that completed the annual;
one issue of John Constantine: Hellblazer (thus, Gaiman wrote two comics about Constantine because this Alan Moore-created character also starred in The Sandman no 3);
Sandman Midnight Theatre, about the Golden Age Sandman meeting the imprisoned Dream of the Endless in 1939.

Thus, these five items represent only three previously published comics. The three issue Black Orchid mini-series would perhaps have been too long to include and also maybe was still in print at the time whereas Gaiman's Poison Ivy Secret Origins story, which tied into Black Orchid, should have been collected in Midnight Days. Gaiman had, at that time, developed a "...vegetable theology..." (p. 16) that would have linked Poison Ivy, Black Orchid and Swamp Thing had he, as intended, become a regular writer of Swamp Thing. Some of the connections are discernible in the works that he did write.

Also:
Wesley Dodds met Dream during his imprisonment;
Constantine met Dream shortly after his escape;
Constantine mentored Swampy;
Black Orchid consulted Swampy.

Thus, there is considerable connectivity between these characters.