Books cracking up bitter pills regularly (8)
I believe the answer is:
libretti
'books' is the definition.
(libretto is a kind of book)
'cracking up bitter pills regularly' is the wordplay.
'cracking up' indicates an anagram (the original order breaks down or cracks up).
'regularly' indicates alternate letters (regularly take one letter, leave next etc.).
The alternating letters of 'pills' are 'il'.
'bitter'+'il'='bitteril'
'bitteril' anagrammed gives 'LIBRETTI'.
(Other definitions for libretti that I've seen before include "Opera texts" , "score-books?" , "Da Ponte's texts" , "Words" , "Books" .)