I would recommend Agenda 21: An Expose of the United Nations' Sustainable Development Initiative and the Forfeiture of American Sovereignty and Liberties by Ron Taylor.
A second book I'd recommend is:
BEHIND THE GREEN MASK: UN Agenda 21
I downloaded it as a kindle e-book.
If you don't have an e-book reader that's fine -- you can get free e-book reading software for your PC, smart phone, etc.