I've just finished " Shadow divers" by Robert Kurson.
ISBN0-375-76098-9 (paperback) OCLC60426012Dewey Decimal 910.452
It's a very gripping true story about a group of divers working at extreme depths ( for the time ) and their encounter with a Uboat found where there shouldn't be one.
Available from all good bookshops, Abe books (second hand ) and the Internet Archive
https://archive.org/ to loan for 14 days. I found it hard to stop reading!
nb Internet Archive is hard to navigate but persevere and you will get a free read.