Safety first

It's very much imperative to understand that plugins have the ability to affect virtually every part of your blog. If a plugin developer was feeling decidedly evil, he/she could create mayhem to your website with a few tiny lines of code. Make sure you trust the recommending source that's promoting a plugin before you install it.

I personally recommend and reference almost exclusively throughout this book http://Wordpress.org/extend/plugins, the official Wordpress.org Plugin Directory. This self-policing community often provides very valuable and detailed pros and cons for almost every plugin, with the ability to sort by popularity, rank, and date released.