A good map of Gran Canaria is KOMPASS 237
A good guidebook is 'Eyewitness Travel, Top 10, Gran Canaria' (ISBN 978-1-4053-5026-6)
Another useful guide book is 'Essential Gran Canaria' (ISBN 978-0-7495-6010-2)
A useful book for walkers written by a British resident on the island is called 'Don't Leave Gran Canaria Until You've Seen it'.
Descriptions of free campsites, in pdf format, at the bottom of the linked page.
My favourite campsite!
The web-based application form for free camping
In case of difficulty with the web-based booking,
the email address of OIAC is:
Site under construction - watch this space!