I think I mentioned above that Dave Arnold's been doing a segment on Classics in the Field on his Cooking Issues podcast.
A recently mentioned book is Ruth Mary Griswold's Experimental Study of Foods published in 1962 so it's less old than @teonzo may have had in mind for this thread but I thought I'd mention it here anyway since it does meet the "free to read/download" criteria. Here's a link to an online version of Experimental Study of Foods.
Edited to add that this is also not a cookbook. It was written as a text for Food Science students. Dave speaks of it as a predecessor to McGee and especially raves about the first sections on "Introduction to Food Experimentation" and "Introduction to Food Science" as well as the last sections on "Food Experimentation." Even if I'm not doing experiments, I'm often comparing recipe outcomes when I change things and found her considerations worth a read and thought others may as well.
If you want to hear the rest of Dave's rave about the book, the segment starts around the 55 minute mark here: Cooking Issues Episode 376: The Army Ant of Grass
