This is part of a state-wide project to index marriages prior to 1900. County volunteers transcribed the marriage books at the County and forwarded them to the State Archives, which entered them into a computer database and produced this volume for DuPage County.
The volumes at the Family History Library and Naperville Public Library are hard copy (and the FHL version has been filmed). The actual book is viewable online in its entirety at the Illinois Digital Archives. It can be searched by name.
Also, the entire pre-1900 Illinois marriage database is also now available online.
FHL film numbers