Peter Kavanagh published
The John Quinn letters: a pandect in 1960. The book consisted of extracts from the letters of the lawyer John Quinn who had corresponded with many famous literary figures such as James Joyce, W.B. Yeats, T.S. Eliot, etc.
The book is a literary oddity not because of its subject matter but because of the way that Kavanagh collected the extracts. He gained access to Quinn's letters in the manuscript room of the New York Public Library. But he was only allowed to read the letters, not take any notes on them. So he transcribed them, from memory, outside the library.
It was a form of spite publishing because the library had forbid the publication of any of Quinn's letters until 1988, and Kavanagh disagreed with this on principle. Also, he published the book on his own handmade printing press. The NYPL promptly sued him and barred distribution of the book.
I don't think it's possible to buy a copy of Kavanagh's book today,
but a few libraries have copies of it. I believe there are only 12 copies of it still in existence.
More details from
Life magazine (Feb 8, 1960):
When he died in 1924, Quinn bequeathed his letters to the New York Public Library, but the courts construed his will as barring publication in any form until 1988. Scholars who have been permitted to read them in the library's Manuscript Room have to sign a special form agreeing not to use direct quotation, and are forbidden to take notes.
But to Kavanagh, these restrictions were outrageously unjust...
In the Manuscript Room, he had no compunction about signing the pledge not to quote from the letters. "To me," he explains, "that paper had no more validity than posting a sign in my flat, 'Not responsible if the roof falls in.' I was driven and had no choice."
For 13 days Kavanagh pored over the letters. Unable to take notes, he simply memorized salient passages, then rushed outside to jot them down. When he had all he wanted he went on to the most arduous task of all: hand-setting the book and printing it...
Kavanagh had not sold a single copy of the Quinn Letters when the library served him with a restraining order, preventing him from distributing the edition and demanding its confiscation. At that point, Kavanagh made a heartbreaking decision.
"I don't want their bloody hands on my book," he said, and on the morning of the hearing he systematically hacked 117 volumes with a shoemaker's knife, shearing them down the middle. "It's like tearing my heart out," he said...
Kavanagh arrived in court with a briefcase crammed with the literary remains. He approached the bench and addressed the judge as "your lordship." Then he upended his briefcase and scattered his shredded copies as evidence that he had obeyed the injunction. The judge explained that he was not "his lordship" and gave Kavanagh permission to keep two unshredded copies of the book for himself.

Windsor Star - Jan 18, 1960
Heinrich Gerlach's semi-autobiographical novel,
The Forsaken Army, recounts events at the Battle of Stalingrad, which he participated in as an officer in the 14th Panzer Division. But what sets the novel apart as a literary curiosity is that Gerlach wrote much of it while under hypnosis.
The story goes that Gerlach wrote the book while he was being held prisoner by the Soviets after the battle. However, the Soviets then confiscated his manuscript.
Years later, after he had been freed and was back in Germany, Gerlach used hypnosis to reconstruct his lost manuscript. When it was published in 1957, it became a bestseller.

Life - Mar 7, 1960
In 2012, after Gerlach was dead, his original manuscript was found in Russian military archives. It was published a few years later as
Breakout at Stalingrad (or, sometimes,
Breakthrough at Stalingrad). So, if you want, you can read both versions.
Somewhat related, the
Guardian has
a list of 10 famous lost manuscripts.
More info:
wikipedia;
The Forsaken Army (archive.org link)
Joshua Alper's 1978 book,
The Documentary Record of an Infringement, documents his "pseudovandalist" alteration of a damaged billboard to make it read "anal Airlines."
Pre-alteration and damage, the billboard was for
National Airlines, which is now defunct.
The book is quite rare,
but you can get a copy for $100.
I haven't been able to find a picture of the billboard post-alteration, and I'm not going to pay the money for his book.
The
Book-of-the-Month Club launched in 1926. New York lawyer Eustace Seligman became the first member.
Life magazine caught up with Seligman 23 years later and discovered that not only was he still a member, but he had bought (and still owned) every monthly selection, plus the various extra books offered, for a total of 449 books.

Life - June 20, 1949

source: Google Images
Seligman died in 1976. I wonder if he had continued to purchase every monthly selection.
So what were the titles of all those books that Seligman owned? Blogger Jeyla Briar has compiled
a list of every monthly selection from the Book-of-the-Month Club since 1926. (Around 2015 they started offering 4 or 5 monthly selections rather than one).
I don't recognize the majority of the titles. But if you're looking for a reading challenge, working your way through every Book-of-the-Month Club selection would be a daunting one.
And yes,
the Book-of-the-Month Club is still going strong, with around 100,000 current members.
This text seems to be missing coverage of fully half of all possible juvenile delinquents. But why not
see for yourself at the Internet Archive.