This collection of Swedish soldiers diaries, letters and other documents was collected in the early 1900s and runs to 12 volumes. It is in Swedish and to make matters more difficult for non-Swedish speakers the spelling is somewhat archaic, but it is a major primary source for historians of the GNW.
It is now online: http://runeberg.org/karolikrig/
Even if you can't read Swedish there are plates and OOBs. Each volume has a link to each page. Look for "plansch" and "utvikplansch."