<p>Say if I had a table of books in a MySQL database and I wanted to search the 'title' field for keywords (input by the user in a search field); what's the best way of doing this in PHP? Is the MySQL <code>LIKE</code> command the most efficient way to search?</p>

Yes, the most efficient way usually is searching in the database. To do that you have three alternatives:
LIKE, ILIKE
to match exact substrings
RLIKE
to match POSIX regexes
FULLTEXT
indexes to match another three different kinds of search aimed at natural language processing
So it depends on what will you be actually searching for to decide what would the best be. For book titles I'd offer a LIKE search for exact substring match, useful when people know the book they're looking for and also a FULLTEXT search to help find titles similar to a word or phrase. I'd give them different names on the interface of course, probably something like exact for the substring search and similar for the fulltext search.
An example about fulltext:
http://www.onlamp.com/pub/a/onlamp/2003/06/26/fulltext.html