That homepage isn't very informative, but you can search for any topic and find explanations and problems that can be solved from that. For example: radical axis [2]
If you're looking for a book, The Art and Craft of Problem Solving by Paul Zeitz is the most recommended.
[1] http://www.artofproblemsolving.com/wiki/index.php/Main_Page
[2] http://www.artofproblemsolving.com/wiki/index.php/Radical_ax...