Thanks a bunch!
So in terms of direct use ── it’s meant to not need anything else to hand (except spell lists for casting classes: I tend to use the little booklets from Lost Pages for mine). But in terms of indirect use, it’s purposefully lean because it assumes anyone buying an odnd derivative will have read (and likely played) at least one or two other versions of the same sort of system.
The only other major oversight within the booklet is that there’s no direct stats for monsters, which is somewhat intentional. I run a lot of horror games, and being a Brit I live in the long shadow of Warhammer, so around 80% of my monsters/NPCs are just Other People. Though if people wanted advice on specific monster books to go looking for, I’d recommend
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Out of the Pit for Advanced Fighting Fantasy et al.
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Monsters from Luke Gearing