There are various types of documentation regarding SquirrelMail. It's recommended to read the documentation maintained by the SquirrelMail Project Team before reading the wiki (which isn't actively maintained). Note that most documentation intended for the end users are part of SquirrelMail itself (along with its plugins), and is easily available by clicking "Help" when logged in.
The SquirrelMail Administrator's Manual
The SquirrelMail Developer's Manual
The SquirrelMail Translator's Manual
The SquirrelMail User's Manual