How do you look up a word while reading a paper book?
There are four realistic options. Type the word into a dictionary app, point a camera app at it, read the book on an e-reader instead, or keep a paper dictionary beside you. Which is best depends almost entirely on how much each interruption costs you, not on which dictionary is most thorough.
Typing is the most reliable and the slowest. You put the book down, unlock the phone, type a word you may not be able to spell, and then find your line again. Readers often stop bothering and guess from context instead, which is a reasonable trade but means the word is never learned.
A camera dictionary removes the typing. You hold the phone over the page and the definition appears without you leaving the camera. Booktionary works this way: rest a ring on the word, a highlight sweeps across it, the definition slides up, and the camera stays open so the next word costs nothing.
An e-reader solves this best of all, with tap-to-define built in — but only for ebooks. For a printed book the choice is between typing and a camera.