Kōya-san (高野山) or Mt. Koya, is an area nestled between 8 Mountains near the border between Nara and Wakayama. It started as a temple in the year 826 and has evolved into a sprawling complex of more than 100 temples. It is the Headquarters of Shingon Buddhism.
Koyasan has 2 excellent websites. The Koyasan Tourist Association, and the Koyasan Shingon Buddhism site. both are packed with information on places to see and history. You can download a better version of the map above at the Koyasan Shingon site.
As I have said in previous posts, the go-shuin-chou stamp book is the best kept secret in Japan. I think it is an excellent and inexpensive souvenir you can bring back from your trip. The journal-like book is a collection of hand painted calligraphy from the temples that you visited, and dates that you were there. You simply hand your stamp book to the friendly monk at any temple in Japan. He then takes your book and paints a unique stamp and date in it. You donate a few bucks to the temple, and by the end of your trip you have this beautiful collection of Japanese calligraphy to take home with you.
I have found a few more comprehensive goshuinchou websites. This one by Waru Emon has a book cover for each temple he has been to.
I want to thank Carol for sending in these photos. It was her comments on the Japanese temple book post that started this whole info seeking journey.