If you've been in this hobby for any amount of time, you've probably heard something about borderlands, most likely from the classic adventure, "The Keep on the Borderlands".
What exactly is a borderlands though? In terms of adventures such as Keep, the answer is... nebulous. Call it the edge of lawful lands buffering the lawless, settled lands on the edges of the wild, the identities of the two sides are very much determined by a particular point of view, that of the "civilized side," usually made up of humans (mostly) and a smattering of your choice of fantasy good folk (elves, dwarves, etc.) The landscape is the same. The difference lies in the inhabitants, the familiar versus the other-- there be monsters yonder, however you wish to define monsters and wherever you wish to place yonder.
In "Three Hearts and Three Lions," however, the borderlands aren't just the buffer between the designated familiar and other of a world, it is a place at which two distinct, separate worlds meet: a mundane world and the fantastic world of Faerie.
For long have these marches been in dispute between the sons of men and the folk of the Middle World, and wars and great sorcerous contests have raged, until now I can but say that Faerie and the Holy Empire both claim it, while neither holds real sway...
The borderlands between them are not just a political or cultural zone, convenient to one side or the other, but a place at which two planes physically merge and fuse together, with all the attendant weirdness that such a conjunction would bring.
...yet in truth I must warn that space, like time, is wondrously affected here by the sorceries blowing out of Faerie, so that often the place where you are bound seems near, and then again dwindles into vast and tedious distances beset with perils, and the very land and way ye go remain not the same...
I have very much just started my read of the book but I'm excited for where it all leads. I know that this was, along with many other volumes, recognized in the famous Appendix N of the AD&D Dungeon Master's Guide, so we know Gygax, Arneson, and company were familiar with the work. I don't know whether any of the borderlands, or the underlying concepts, were based upon this book but I wish what we had gotten had preserved more of this flavor.
I am a big fan of Faerie, as the strange and odd maelstrom from which all manner of the fantastic is birthed and this conception of both Faerie itself and this landscape that connects it to "our" world fits in exactly with the sorts of worldbuilding I've been interested in lately. I can't wait to read on and see how the story unfolds.
Obviously, I am super late to this particular party but, as the saying goes, better late than never! Now that the book is in hand, I don't think it will take me long to ingest it and I am looking forward to that journey! Onward, Papillon!