Hi HN, Cycle Archive is a free, browsable library of old cycling magazines and books from the 1860s–1940s.
I started it as a way to explore cycling history myself, and all of the weird and wonderful articles in there. People inventing brakes, fighting about wheel sizes, pioneering indoor training and some pretty questionable dietary and training advice!
The site attempts to pull it all together somewhere you can actually sit and read it. (Almost) every issue has a generated contents list: article titles, a one-line summary each, deep-linked to the exact page in the PDF, with full search across the whole thing. Search is a WIP but it's already quite powerful. Next on the list is making old ads searchable! It's admittedly pretty niche, but would love any feedback or ideas.