I've been on a bit of a binge since Fall acquiring rulesets for TTRPG games that I find interesting. I watch a few online channels fairly regularly, and reviews come up for various systems and modules in the realm of OSR. I've also shopped at DriveThru RPG in various categories looking for rules light systems that I can implement in a solo player format.
While it is nice to be able to purchase a rule set in pdf and receive it immediately, there is just something to be said for having a hard copy you can hold in your hand. I have the pdf for all these volumes, but I enjoy getting the booklets when I can.
I searched a little for an original copy of Chainmail to go with my OD&D collection, but really couldn't justify the cost. The print on demand version works well enough for my needs. Who knows? I may find an original at an estate sale someday. I purchased White Box to cover OD&D basics so I can reduce the wear and tear on my first three original volumes. I have the Old School Essentials Basic, but only in printed pdf form, and the price was really great for the White Box rule set.
I purchased Mork Borg at the local Gaming store and got the supplement off of Amazon. Their site has pdf support so I can use that for reference as opposed to thumbing thru the rules book, but I'm glad I bought the hardback as it really is a unique project, worth the bound price.
Both of these books are rule sets by Questing Beast, and are in the rules light category of OSR.
I printed out Maze Rats from the pdf as I didn't have any luck finding a copy, or a print on demand version. Knave is a newer release, and was still available in print.
Another rules light OSR that I found intriguing is DEATHBRINGER by Dungeoncraft. No spells are listed in this rule set so you need to source them elsewhere, but this set is a quick read, printed double side on a single sheet of paper folded into a booklet. Well worth having for the price point.
On the way is Solo Gamemasters Guide by Geek Gamers and the Master Edition of Index Card RPG.