In my childhood, I lost myself in the worlds of Enid Blyton, Ramona Quimby and later, dare I say, the characters of V.C. Andrews.  I would spend hours upon hours on the floor of my room flipping pages, recalling the smell of a new book from Coles (my favourite store at the time) and any book which included some type of map in the front cover so the reader could understand the geography of the time. 

I recall being called a 'bookworm' by my mother and simply understood this to mean someone who read A LOT.  It turns out I was not entirely correct.  Here are three sources providing an explanation as to the origin of the idiom:
 

If you love to read books, you may have been called a 'bookworm'. The origin of the idiom “bookworm” probably originated as a somewhat derogatory term for a person who studied or read more than was usual. Bugs such as silverfish, book lice, and linoleum beetles were referred to as bookworms because they inhabited books; thus the idiom.

https://www.gingersoftware.com/content/phrases/bookworm/
 

The classic dictionary lookup:
 

noun

a person devoted to reading or studying.

any of various insects that feed on books, especially a booklouse.

https://www.dictionary.com/browse/bookworm
 

Lastly a look at the negativity of the term turned positive - follow the link at the end for the positive note
 

The most literal meaning of bookworm is also its least common one: a bookworm is the larva of an insect that causes damage to books. Not so literally, a bookworm is a voracious reader of books. The entomological word can refer to one of several insects, especially the drugstore beetle and the cigarette beetle. These creatures like to dine on bookbinder's paste, bindings, and the leaves of books.

Originally, 'bookworm' was an entirely negative term: 'worm' was an Elizabethan insult that meant "wretch," and to be called a 'bookworm' was an insult

https://www.merriam-webster.com/words-at-play/is-the-meaning-of-bookworm-positive-or-negative