When I started out playing and DMing Old School Renaissance games I downloaded a bunch of free pdfs (usually the art-less versions of games, or games that were completely free - no pirated games). Delving Deeper, Labyrinth Lord, Whitebox, and a few others. I've thumbed through and read most parts of most of those, but quickly settled for running Labyrinth Lord almost exclusively.
The game is complete in itself, and the art-less edition is pretty as is. Also, I had it printed out in A5 format and glued a soft spine onto it. It's small, it's inexpensive, it's tea-stained. It took me embarrassingly long to figure out that I could just write stuff in the margins or onto the pages where needed instead of cross-referencing stuff. But I love that book.
Also, I only use a tiny portion of it. I very much like to invent my own monsters tailored to my own adventures. I exclusively play OSR on conventions with a huge majority level 1 PCs, which means I've hardly ever used a spell above level 2. Spells is also one of those things that I like doing my own to sprinkle out as treasure. And I've never used the treasure tables, or the instructions for building dungeons, or the magic items. Two days ago was actually the first time I even read through the magic items chapter, which was a bit of an eye-opener but not enough so that I'll use items straight from it. I don't care about class prerequisites or modifying XP according to primary abilities. I've never had use of either maritime or aerial combat. To summarize: I only use a small portion of the book. Almost just a tiny portion, actually.
For years I've been thinking about writing my own OSR game, which I guess would just be a summary of the rules I use and perhaps a few house rules. Not sure where the threshold is for when a project like that is worth the effort, though.
-- CC0 Björn Wärmedal