There are 2 ways to search a word within a book:
(1) Selecting a word or a string of words (by touching and holding it), you will then see an option menu showing up. Selecting the "Search" icon.
(2) Tapping on the “Search” icon at the ActionBar, typing a word or a string of words in the search box and then tapping on the search icon.
After search, you will see the list of pages where the words appear. Tapping on a particular page will bring you to that page and the words will be highlighted.