The most common and most likely answer to this crossword clue is the 9 letter word NONREADER
I've seen this crossword clue in The Times
N
O
N
R
E
A
D
E
R
Buddy explains!
The straight part
We know that "One won't be able to understand characters in a book" is the straight/definition part of the clue. The straight part is often a synonym or definition, in this case one won't be able to understand characters in a book = nonreader.The cryptic/wordplay part
In cryptic crossword there's often an wordplay/cryptic part as well.This part involves a more complex hint, such as an anagram, homophone, hidden word, etc.
Sadly, We don't have an explanation for this specific crossword clue yet.
What does NONREADER mean?
NONREADER noun- One who does not read.