Guidebooks

One of the best book in Japanese is “Hokkaido no Yama”(2010 Yamatokeikokusya “Yamakei” ).
Which illustrates more than 50 mountains, and its descriptions are very comprehensive. Map, route and standard walking time are desipherable.
  Amazon.co.jp ships this book internationally.

(2010/02/22) Kenji Ito check this book

In English, “Hiking in Japan”(2009 Lonely Planet) illustrates 14 routes in Hokkaido.

(2009/08) Richard Ryall check this book

Websites

Using Japanese names mentioned in Mountains Overview as the keyword, you can search at;

tozan-guide.com
unfortunately is only in Japanese as of Jan.2014, but plenty of photo, movie, access, route, hours, hut, several rankings are helpful to know each mountains.

yamareco.com
is the archive of the each climbing records so detailed more than tozan-guide.com.

mountrec.com
is a subsidiary site in English of yamareco.com, just started to collect records.

Preparation

The maps can be obtained at;

watchizu(GIAJ)
is Geospatial Information Authority of Japan, 1/25000 maps are available for checking terrain and path.

Real maps are available in Sapporo,

Kinokuniya bookstore
at Sapporo Sta. or,

Syugakuso
at “Kita 12(Juu-ni) Jo” subway station next to Sapporo Sta. Monday closed.
The stores at Shiroishi in Sapporo and Asahikawa too. All mountain gears are available there.

You would better to consider that after leaving Sapporo the chance of replenishment are supermarkets, home improvement centers and convenience stores.