I have not read "Iron Coffins", so cannot comment on it, but personal account or memoirs are often very subjective accounts. There are many reasons from this, not all of which are attibutable to outright lying.
1. It is based on fog of war and incomplete info. Many sub skippers on both sides reported hits or sinkings which made sense at the time, based from their viewpoint, but which could not be justified by postwar records;
2. It is based on memory. Unless someone took copious notes or rechecked the records, writing down an event years afterwards, just from memory is rarely accurate. Events get mixed up, certain aspects are forgotten, etc.
3. self-justification. more nebulous concept, but people have a hard time taking an objective look at themselves and will usually try to rationalise, justify or present their actions in a positive light.
I usually stay away from memoirs for those reasons and read thoroughly researched history books, like Blair's books:
"Hitler's U-Boat War" and "Silent Victory" on the pacific submarine war.
On the wider concept of the romance of submarine warfare, I have always wondered about that as well. If you look at it objectively, the whole concept of a submarine in WW2 (whether U.S. or German) was to carry out sneak attacks on civilian ships which often resulted in the deaths of men, women and children. That is about as glamorous as being part of a bomber crew bombing enemy cities or snipers/sharpshooters in a land army.
I presume the "romance" comes directly out of WW2 propaganda. The Nazi regime glamourized U-Boat skippers and their exploits, painting them as aces and heroes. i.e:
Even Allied propaganda painted U-Boat skippers as "dangerous" and "pirates" which gave them a glamorous anti-hero image, much like U.S. gangsters in the 20's and 30's. For example:
