There are a number of interesting historical references on the web for brewing beer. They come from books that are old enough that the copyright has lapsed, and as a result the books have come into the public domain. So we're talking about books that are 100 years old, or more. I found that some of them had interesting info, or interesting illustrations, but unless you're primarily interested in the history of brewing you're better off getting a book that was published in this century.
Some of these old books have been converted into electronic versions by Google or similar projects. If you want, you can also buy the paperback versions of some old brewing books on Amazon. For example, you can get various re-published books by by Marcus Lafayette Byrn on Amazon, such as The Complete Practical Brewer; Or, Plain, Accurate, And Thorough Instructions In The Art Of Brewing Ale, Beer, And Porter; Including The Process Of Brewing Beer, Ginger Pop, Sasparilla-Beer, Mead...
You can also find electronic versions of various works Marcus Lafayette Byrn and other authors for free. The quality varies. You can usually download them in a variety of formats. but Project Gutenberg has the most options. Most of the books originated in the US Public Library or University library systems, so you'll often find library marks in the scans.
Project Gutenberg books:
A Treatise on the Brewing of Beer by E. Hughes
The London and Country Brewer by Anonymous
The Closet of Sir Kenelm Digby Knight Opened by Kenelm Digby. Includes recipes for mead and metheglin.
The American Practical Brewer and Tanner by Joseph Coppinger
The Complete Practical Brewer by Marcus Lafayette Byrn
The town and country brewery book: or, Every man his own brewer By W. Brande, originally published in 1830
The brewer's analyst: a systematic handbook of analysis relating to brewing by R. Douglas Bailey, originally published in 1907, this book details how to analyze beer using archaic instruments.
The theory and practice of modern brewing: A re-written and much enlarged by Frank Faulkner originally published in 1888
Brewing by Alfred Chaston Chapman. The cover illustration is a 'vertical refrigerator' AKA wort chiller.
A Practical Treatise On Brewing, Based On Chemical And Economical Principles : With Formulæ for Public Brewers, And Instructions for Private Families by William Black, Practical Brewer
The Theory And Practice of Brewing by John Adams. Due to fonts and typography a century ago, the letter "s" was printed differently when it appeared at the beginning or middle of a word, instead of at the end. The OCR engine does not take this into account, so you'll see lots of words like "fiences" rather than "sciences".
Laboratory Studies for Brewing Students, a Systematic Course of Practical Work in the Scientific Principles Underlying the Processes of Malting And Brewing by Adrian J Brown
The Brewer's Analyst: a Systematic Handbook of Analysis Relating to Brewing And Malting by R. Douglas Bailey
A Text-book of the Science of Brewing by Edward Ralph Moritz , George Harris Morris. Illustrations are missing