Archive.org has digitized a great number of books related to Bronx County. I've selected a number of them to highlight below. Clicking on the book title will take you to the on-line book to read. Clicking this link will take you to a listing of many more books than what we've highlighted on this page.
The Borough of the Bronx, 1639-1913 by Harry T Cook, 1913
History of Bronx Borough, City of New York by Randall Comfort, 1906