Hello,
If your ds normally takes 20 mins to drain one boob, if you only offered him both boobs for 2 x 10 mins, then the first boob won't be empty. You need to make sure that he drains one boob before offering him the other as it is the hind milk that is the most calorific.
The book is just a guide - all babies are different. My dd drains one boob within 5 mins, she is an efficient feeder.
Sounds like your baby is doing really well!
Kat x