If you load books directly from Calibre there will be folders sorted by Authors last name. In the Library (accessed easily by the home button) there's a choice of "All "Books" "Shelves" My Files". You'll find this under:
My Files -> My Files Folder ->Books folder (same with main Nook or added card)
You can also just do a search for a book title or author with the Search Function - accessed by the Nook home button.
Because I drag around so many books and different authors I like to sort my books by genre. (This doesn't affect the other sorting options of "Author" "Title" "Most Recent" the Nook offers.) I tag my books on Calibre but The Nook can't read tags unless it's rooted. So instead of sending my books to the Nook via "Send To Device" I created folders by genre on my Nook (and the card) under the Books Folder and sent the books by genre choosing "Save To Disk" and "Save Only To Single Directory " The big drawback is not having the convenience of sorting by multiple tags since some books cross genres.
In the directory that's
Nook -> MyFiles -> Books ->
I created sub folders "Mysteries", "Historical Fiction", "Fantasy" etc
If you don't use Calibre you can just move files into these sub directories using your computer's file system.