So I'm a new poster here, but I've been browsing this place (including the old website, I believe), the mimir website, Wandering Genasi Tavern, pathguy's planescape section, and a number of other websites for years. Despite this I have only managed to get one actual Planescape book, The Planar Handbook for 3.5
Anyway, I've been strongly considering DM'ing a Planescape game for 5e lately, but I feel like I've about exhausted most of the online resources I could find for ideas. Since I couldn't decide what to look further into, I figured I'd ask you guys, seeing as how this seems to be one of the premier places for Planescape knowledge. Are there any Planescape books (for any edition) that you think would be useful for running a game in 5e that cover things that aren't already done to death online? As well, do you guys know any of the more reclusive Planescape websites in various corners of the internet that I might have missed that would be useful? I'm especially interested in the inner planes (particularly the quasi, para, and similar areas) and planetouched, but other information is useful as well. Any suggestions are welcome.