Those are some funny stories. Both Last Onslaught and Built to Better the Best are fine references with different viewpoints. However, BOTH are considered old school books and somewhat out of favor with what some new readers want.
This may be one reason that both have seen declines in prices. Last Onslaught takes some focus to read the fine print and the photos are mostly archival black and white and do not "pop". Nowadays, they have high resolution photos, the set on the pages is more user friendly, the quality of the paper stock is superior, and so on.
Plus everyone is aging and fewer folks want these books and KF books in general. I think the writing of historical books on the automobiles we love and the history of the men and companies is far less than it should be, which is a shame.
I have read with gusto many books on old cars-history of companies. One of the best is Robert Lacey's "Ford - the Men and Machines". Langworth's book "Last Onslaught" is special because it is not just reporting, there are interjections of humor and antecdotals.
I have gotten into and out of this old car hobby a few times in my 53 years, and when I do I give away my books. I know some people think they are super valuable and I should throw them on eBay, but I just give them away. Gave away my Built to Better the Best and tried to give away my Collectible Automobiles although I still have a stack I need to throw out including the very 1st issue.
But I retain my Last Onslaught because it was a Christmas gift from my wife and is signed by Langworth. I began reading Langworth's work in 1978-79 when he was writing for Collector Cars magazine, which I believe is no longer published. It was an article on the 1949 GM hardtops (ironically).
Another book I should have kept was the "pictures" book, the photographic based one. Lots of great photos of the Virginians. Maybe I will buy some again.