A possible problem is that it has to be an exact match. If there are non-display characters within the word, such as a soft hyphen, the search won't match. Or if for some reason, the word is broken by tags, it won't work. If for some reason the book had:
The search would not find the word "footbridge". And while I would be surprised to see the above code, though only a little, I have seen something like:
Code:
<i>this</i> <i>is</i> <i>in</i> <i>italics</i>
That will look fine, but, searching for "this is in" will not find the above.