Hathi Trust Full Text Search
Search HathiTrust (pronounced hah-tee) like you would any library catalog.
Select the "full view only" only box to return results that are entirely free and full text. If you don't select the box, you will get items with a limited preview, but you can then order those titles from Interlibrary Loan.
For more refined results, you can explore collections (19th century cookbooks, or 18th century Travel in England, for example)- and search within them.