For starters, "Beowulf" is being lauded, extremely good book (5)
I believe the answer is:
bible
'good book' is the definition.
('bible' can be a synonym of 'good book')
'for starters beowulf is being lauded extremely' is the wordplay.
'for starters' says to take the initial letters.
The initial letters of 'beowulf is being lauded extremely' is 'BIBLE'.
(Other definitions for bible that I've seen before include "An authoritative text" , "It contains scriptures" , "Book of the Scriptures" , "Book for someone swearing?" , "Book regarded as the ultimate authority on its subject" .)