Is there an app that gives you Kindle's tap-to-define on paper books?
Tap-to-define is the feature people miss most when they go back to paper, and nothing reproduces it exactly. A printed page cannot report which word you touched, so the phone has to read the page and work out which word you mean.
Camera dictionaries are the closest equivalent. You hold the phone over the page and either tap the word on screen or hold a marker over it. Booktionary uses the second approach: a small ring stays in the centre of the frame, you put it on the word, and about a second later the definition appears.
The practical difference from a Kindle is that you hold a phone above the book rather than touching the page, and that lighting matters. The practical similarity is that the lookup no longer interrupts you — the camera stays open, so consecutive lookups cost nothing.
Booktionary keeps both dictionaries on the device, so unlike a Kindle it needs no connection at all. English and French, $1.99 once.