I don’t really use Apple Mail myself — these days I do all my email via the Gmail web interface. Plenty of people out there do use Mail.app, and may find the Take Control of Apple Mail in Leopard ebook useful:
You’ll learn how to use and customize the Mail window, control the size and styling of incoming messages, and make rules to move messages into different mailboxes automatically. The book covers outgoing mail, showing you smart ways to address messages, send attachments, and send HTML-based messages. But, that’s not all! You’ll also find advice about setting up accounts, solving account connection problems and other bug-a-boos, handling spam, managing attachments, making backups, searching, signatures, notes and to-do items, Data Detectors, and more.
Book Info: 95 pages; Published 13-May-08; 1.2 MB download; Free sample with Table of Contents, Introduction, Quick Start, and section starts. US$10.
I’ve bought and read other Take Control books before now, and they’ve always been well-written, well-produced and worth the cost.