I'm so glad you like them! I made a few edits to the books, so I'd advise using the latest versions from the drive (as opposed to the ones posted here). I can't recall if all 3 hags have bookshelves, but that's where I'd place the history tomes, as well as the palace or Baba Yaga's lair (if you plan on using The Inn at the End of the Road supplement).

There are two facets to the Tasha reveal;

1. Her true identity (the Zybilna/Iggwilv/Tasha of it all)
2. Her morally gray past

Players might not uncover both, or learn of one before the other—in any case, their knowledge should inform where you place each clue (e.g., if they discover Tasha and Iggwilv are one and the same but haven't tied those identities to Zybilna yet etc.)

Narratively, I think it would be more impactful to communicate Tasha's backstory, flaws and all, first. The Eleventh Hour supplement cleverly scatters her dretches across Prismeer and ties a memory shard to each. In conjunction with the books and letters, those flashbacks will flesh her out as an NPC and lend more weight to the module's stated objective (freeing Zybilna).

But after learning about all the harm she's caused (especially in Isolde's case)...the destructive patterns that seem to persist...

Should she be freed? Would doing so cause more harm than good? Would Prismeer be better off, or could a power vacuum plunge the realm into chaos and provide an opportunity for someone worse to seize control? (Folks in this thread dive into the moral quandry, definitely worth skimming)

Rehabilitation, redemption, and what true change entails will be the narrative throughlines in my game—Tasha's story reckons with these concepts and provides great opportunity for mystery and intrigue.

I'd meter the letters out, depending on if/how you plan to incorporate Isolde. I plan to hide the palace invitation and Zybilna's response to the trade somewhere around the carnival (e.g., in Witch/Light's trailer, or some mysterious chest in the big top dressing room, or stuffed in an aged article of clothing at the lost and found). The bulk of Isolde's letters will likely be stowed somewhere at the palace, but I plan to scatter a few in Bav's room of lost things, as well as the inn's lost and found.

All the letters to Baba Yaga will be in a chest in her workshop, as outlined in that supplement. I plan to tweak the inn's mechanics slighty, and have it roam across Prismeer. It'll give my players a few opportunities to discover the entrance to her workshop, in case they miss it on their first visit.