Handling the big jets is nice but if you want an answer you just might start with "Gas turbine theory" by Cohen, Rogers and Saravanamuttoo ISBN 0 582 44927 8 If you have any questions after that there is a nice load of literature mentioned in it.
BTW it has something to do with cold air and thin air. hence the optimum around the tropopause.....
good luck and a lot of reading pleasure.