A very readable account of the exploits of a then twenty-six year old commander of the 600 tonne (relatively small) submarine Unbroken in the Mediterranean during WWII.
There were a range of duties, including convoy protection, attacking merchant shipping and enemy ships, and some special operations including landing commandos in France and North Africa.
One thing that was quite remarkable was the use of the submarine to attack infrastructure on the Italian mainland - trains, railways, viaducts and such like. I've not seen mention of this in other books on the subject, but it seems to have been relatively successful, with the aim of stopping supplies getting through to troops in North Africa. It is difficult to gauge its extent and significance.