It's been a while since I posted here but (hopefully that will change).
I can report a small update in identifying the Edward Joseph Jekyll named on a Victorian bookplate (see HERE for the original posting).
I was leafing through Harrington Manville's book 'British Numismatic Auction Catalogues, 1710-1984' when I came across a sale catalogue of Sotheby's from 15-16 June 1915 which listed the collection of early British, Anglo Saxon and English coins of Edward Joseph Jekyll of Ampthill.
As the younger Captain Edward Jekyll died in 1921 I suspect that this is his collection (and therefore he was the previous owner of the book in the original post). That is not to say that any collection or library of the elder Edward Jekyll wasn't passed down to his son.