For PG books, the simplest way is to go to the home page at <http://www.gutenberg.net>, type the Author or Title into the search form, press the "Search" button, and follow the choices.
More finding and browsing options are available at the "Find an eBook" page at http://www.gutenberg.net/find
There is a full-text search available at
where you can search not only for titles and authors, but any words or phrases you want to look up. For example, entering "Ample make this bed" and running an "entire books" search for all words leads you to Poems Of Emily Dickinson, Series Two. It does, however, lag behind, since it must be rebuilt periodically. At the end of 2003, it is about 5 months behind. While search engines like Google do reference our texts, they typically catalog only the first 100K or so of each file, so if you're searching for a quote near the end of a book, they may not find it.