I noticed quite a large amount of AD&D books at Barnes & Noble, and that got me thinking: Are there any good books with any relation to PST? e.g. Sigil, Lady of Pain, etc.

Wow, so a little searching later and I've found at least one good possibility:

http://www.amazon.com/Pages-Pain-Planescape-Troy-Denning/dp/0786906715/ref=pd_sim_b_3

Really interesting idea there as I've always wondered more about the Lady of Pain.

Also these might be pretty cool, too:

http://www.amazon.com/Blood-Hostages-Wars-Trilogy/dp/0786904739/ref=pd_sim_b_1
http://www.amazon.com/Abyssal-Warriors-Planescape-Blood-Trilogy/dp/0786905018/ref=pd_sim_b_1
http://www.amazon.com/Planar-Powers-Planescape-Blood-Trilogy/dp/0786905328/ref=pd_sim_b_1

There actually is a .pdf book included as a bonus in the GOG PS:T package. Written by the author of the game Chris Avalonne. Check it out!

It's not written by Avellone per se, it's an adaptation of the story he and the other writers wrote for the game. It's an alright book in relationship to the game I suppose, but it's not a good read unless you've played the game. PST being a game and all, the book has a very weird structure as a result. It's more of a compliation of the games dialogue sections than anything else.

This. All novel adaptations of the game are terrible for varying reasons, and this one not even trying to actually adapt the material to a new medium makes it an especially painful read. It's either that or the novel that came out back when the game did which commits so many grave errors with the changes made to the plot (some of which were so bad that the wikipedia summary alone contained enough concentrated wrong to form a black hole), or the ShadowCatBoy adaptation that has been posted in this form that was a piss-poor attempt to adapt a SA LP thread of the game that had only a passing familiarity with spellcheck and basic writing concepts.

Apart from Torment, Pages of Pain was the only other book I knew of set in the Planescape universe, the latter which had fairly critical reviews. I've not read Pages of Pain myself but the critics of the book complained that it destroyed some of the mystery behind the lady of pains character.

I would like to see some other books in Planescape but unfortunately, it's all about the Forgotten Realms and Dragonlance. Whether Pages of Pain was good or bad, I still appreciate it that Troy Denning took the time to write a novel with the Planescape name on it. Not sure about the other 3 you mentioned in your first post but I'm tempted to hunt them down.

Really hope to see more authors look at the Planescape setting a little more

Oh for the love of... The Lady of Pain is. That is all that should ever be known. She is the last great mystery of the multiverse, as the gods in Planescape have addresses, stats, specific limitations on their power the closer they get to the spire; they may be powerful, but their power is definable. The Lady of Pain is the one thing that can defy the powers with no consequences and do a host of other impossible things, and noone is sure how exactly she does it or how far her power extends. The only way someone could miss the point of Her Serenity is if they made her defeatable by the PCs in a game or some piking Clueless protagonists in a book.

In addition, the deity's you mention also draw their power from believers. The more worshipers a deity has, the greater their power. The Lady of Pain as you know, has no worshippers or temples dedicated to her which is something else that sets her apart. Kinda makes me wonder how the hero in the Page of Pain book is supposed to find her (talk to random Dabus maybe?) She is meant to be a total mystery that defies the laws as we know them, so when I read the description "Here, in her own words, is her eternal story.", I face palmed so hard.

Also, the book is meant to be about some god sending a random prime to send a trap to the Lady of Pain? There's just so many things wrong with that (one mentioned above).

The Lady of Pain says nothing. The closest she generally comes to communicating is casting her shadow over people to show her displeasure with them. The *one* time I can remember the Lady "speaking" was in the module Faction War when she telepathically (I guess) used a Dabus to communicate her will to the crowd, and that was in the middle of a conflict that threatened to tear the Cage apart. She doesn't exactly seem like the kind of cutter to pour her mysteries into a sodding diary on a whim like some piking addle-cove. Seriously, what the hell?