These give full date of death and exact place of death and generally provide the names and addresses of the heirs and other relatives of the deceased party. Later records even include detailed questionnaires about wives, children, and other subjects relevant to the estates.
Boone County Probate Index, through 1962, is available at the Belvidere Library, the Family History Center, and the Rockford Library.
Winnebago County Courthouse has two "A-Z" card indexes and a computer with certain years indexed. These are available in the Probate Office (part of the Circuit Clerk's Office)