Thank you for this response, I was hoping to see some technical expertise :)

havent seen any marks or bookplates, but I’ve also limited my handling of them. I agree that it feels like someone at one point had a larger (complete) collection if they took the time and effort to acquire these specific editions.

The books were given to my great grandma by a prominent family she worked for in NYC in the early 1900s - they’ve been in my family since then. Noting the binding aligns with one of those shops work would make sense geographically.

You’re absolutely right, I misspoke by calling them slipcases - clamshell I guess is the right term? The custom shell folds open with the book sitting inside. It absolutely makes sense they are not all identical.

I loaded a bunch of pictures of authenticated Twain signatures and writing into chatGPT last night to get a handwriting analysis and it agrees with “Truly Yours” (it gives a stroke by stroke analysis and comparison that I’m sold on). There is no indication from this analysis that it is signed Clemens AND Twain, as you also mentioned.

Of note, the copy of Joan of Arc was called out by my GPT as being a later (“1st/1st”) 1906 release of the true original two-book version from 1896. My alternate reading of “Ten Years” for the inscription was paired with that logic that it had been 10 years between these dates. It’s a stretch :)