Reference book's style half-dreary (10)
I believe the answer is:
dictionary
'reference book's' is the definition.
(dictionary is a kind of reference book)
'style half-dreary' is the wordplay.
'style' becomes 'diction' (diction is a kind of style**).
'half' suggests halving the letters.
'dreary' with half the letters taken is 'ary'.
'diction'+'ary'='DICTIONARY'
(Other definitions for dictionary that I've seen before include "Ordered book" , "glossary" , "Brewer's creation, say" , "reference work" , "Lexicon" .)