The following tips are from support people who have helped users to understand Confluence search results.
Searching with quotes, for example searching for the string "Book One", does not look for an exact match like most search engines do. Rather, it looks for "Book AND One" and would return all matches where "One" is the first word after "Book" other than words considered to be stop words (a, the, an, etc.) This is explained in the following support request: http://jira.atlassian.com/browse/CONF-7148
It's not possible for Confluence search to return hits for "the best," as an example--with or without quotes, "the" is ignored.
On the back end Confluence is using a search tokenizer called Lucene 
As part of its tokenization process, Lucene strips out all punctuation from the search string, including _ characters.
