Here are some examples of book words: awry, bedraggled, biopic, cache, calliope, Greenwich, Hermione, misled. So, what do these words (and a few dozen more) all have in common?
The answer is that they all are (or might be) words that you mispronounce because of how you learned them: by reading a book instead of by hearing someone say them. Those of us who are exceptionally bookish learn many words that way, guessing the meaning of an unknown word from context without bothering to look up the pronunciation. Naturally—English spelling being what it is—we sometimes get the pronunciation wrong and then never hear the word spoken aloud and therefore never get corrected. Newly famous long-term Jeopardy champ Amy Schneider provided one of my favorite examples of this, misled—favorite because I too had once thought that it was the past tense of an unfamiliar verb “misle,” thus pronounced MY-z’ld, as Amy had thought too).
Anyway, watch this four-minute video from distinguished Canadian linguist James Harbeck, explaining this phenomenon (and concluding with the phrase “reading is good,” just in case you’re thinking that either he or I might be dissing reading). Maybe you too have encountered some book words in your experience learning your native language.