The contact book fills itself: everyone you write to and everyone who writes to you is there, with how often and how recently. Saved holds the people you added or edited yourself.
The book
Search by a part of the name, the address or the company, and sort by name, by frequency or by date. Open a contact to see all emails with them, add a company and a note, or remove them.
The shared directory
Shared holds addresses the whole installation sees: teammates and shared mailboxes appear there by themselves, and an administrator adds addresses everyone needs, such as the bank, accounting or a supplier. The To field suggests them to everyone, marked as a shared entry. Share with all moves one of your contacts there.
Teammates and shared mailboxes can be hidden from the book with Show teammates and shared mailboxes.
Import and export
Import vCards takes a .vcf file from another program; Download vCards gives your book as one .vcf file.