Internet Archive hosts a treasure trove of lost Lego-building instruction booklets

Now this is a helpful treasure trove—a library of Lego building instruction booklet PDFs. It's housed at The Internet Archive, "a non-profit library of millions of free books, movies, software, music, websites, and more." The LEGO library is described on the Internet Archive as "a dump of all available building instruction booklet PDFs from the LEGO website (As of March 2023)"—there are 6,854 instruction booklets included.

Open Culture explains:

If you're missing a few bricks, you can always turn to the robust secondary market in LEGO components. If you're missing the manual, there's now one place you should look first: the LEGO building instructions collection at the Internet Archive. There you'll find digitized materials for more than 6,800 different sets, including such popular releases as the LEGO Chevrolet Camaro Z28, the LEGO International Space Station, and the LEGO cover photo of Meet the Beatles.

My son was obsessed with LEGO his entire childhood and youth, and try as we might, we didn't always successfully manage to hold on to every instruction booklet that came with his sets. This archive sure would have come in handy during those years!