A puzzle

BH is rather fond of a series of about a dozen or so books, the 'My Friend' books, set in the Scottish highlands, at the time the author was a child, say the 1920's. More specifically, Black Isle, which appears to be the peninsular which lies between the Cromarty Firth and Moray Firth, a little to the north of Inverness. The far north. One can usually get these books for less than a fiver a pop, including package and postage, from the efficient Abebooks of reference 1 - once proud to be independent, now part of the Amazon empire.

Recently, she has found another series of four or five books by the same author, but written under the name of Janet Sandison and set in a small town on the outskirts of Glasgow, where her father was stationed as a policeman. A rather less sugar-coated series. Curiously, while most of these books come in, once again, at around a fiver, the last one seems to fetch much larger prices - with that snapped above being a bit extreme, but I could find nothing under £50. 

With prices on eBay being much the same. Clearly the people selling secondhand books do their homework on prices.

So why is this? What is so special about this last book?

Reference 1: https://www.abebooks.co.uk/.