Originally Posted by earthboundagain
it is hard for me to catch him with some free time
Try to engage him by email and ask him (in his free time) to email you some recipes, pieces of advise, tips/tricks that you can share with others so that the knowledge lives and grows. Tell him it's nothing pressing, take his time. Just when ever he gets a chance.
(Stuff like that stirs people. Mostly because they know it to be true.)
Or you can try to swing a "Hey lets write a book".