This was hard today I thought.
Not really impressed with 15D although I managed to parse it... using without to mean "around" does not come up very often I guess. And function is a bit vague really. If it had mentioned Napier or something that would have helped.
8A was a bit vague as well I thought although once i checked what shy as a verb can mean it made sense.
Favourite was probably 11A (that was a bit vague too though, had to have all the crossers for the second word before I got it.
I'd not heard of the book for 1A.
I can't parse 18D either looking at it.