[mistwang]

You need to remove the leading "/" of the matching URL when use a rewrite rule in .htaccess. It is the RewriteBase in Apache. Your rewrite rules must be used in Apache httpd.conf before, not in .htaccess.
Please try,

Redirect permanent ^webmail/ http://mail.server.com/webmail/
Redirect permanent ^Books/ http://www.server.com/books/