General Overview
Knave is the Old School Treasure Hunting RPG from Ben Milton of Questing Beast fame. I give it overall high marks for just being blunt with the readers about the social expectations baked into the game. Rather than beating around the bush it just flat out states how GMs and Players should act while using the rules or they may as well not even bother. Things like GMs are expected to signpost danger, keep time records, and let players guide the action. Players are supposed to take initiative, scheme, fight dirty, and prepare to die.
The book is filled with useful and inspiring d100 tables.
A Mechanic that works really well
If Ben had fully committed to it I would have said "Reversing Checks", something I've always wanted to throw in a rulebook but also always backed away from at the last moment because it feels against tradition. But especially for a game like Knave with a focus on speed of play this works well. The GM ceases to be a bottleneck with rolling and it give players something to do on their turn (which is why I use opposed rolls in NGR). For those unfamiliar, it means instead of the GM rolling a die to get a number to compare against the player's AC which is a flat number, the GM gives a flat number for the attack and the player rolls a d20 with a bonus for their armour to avoid it. So instead of rolling 10 goblin attacks against AC 18, the GM says "You suffer 10 goblin attacks at +0 doing d6 per hit". The player then rolls 10 defenses with a +8 AC bonus and for each failed defense roll they roll their own damage. Meanwhile the GM has already made their way through the rest of the table in the same way. It is listed as an optional rule though.
So instead I will list how Knave uses inventory slots as a "Death and Dismemberment" table. When characters run out of HP (or takes unavoidable damage) they take damage as wounds, each of which takes up one of the approximately 10 inventory slots a character has. They then have to drop things, which is important as Knave is very inventory punishing. Most abilities and powers are item rather than class based (to say nothing of treasure). This creates interesting choices on the way out of the dungeon on the right mix of treasure and tools and what you drop in what order.
A detail that lifts above its weight
Falling is the classic d6 damage per 10 feet. Nothing special there, but there is an added rule where if you roll three 6's for fall damage at once you just die, broken neck. It makes falls dangerous even for high level characters. This makes pits and ledges a barrier even for higher level parties.
Constructive Feedback
Watches (basically wilderness turns) are 6 x 4 hour blocks. Three marching through hexes, three sleeping as a normal bit with options to use a watch for searching a hex for hidden things, foraging, etc.
This is fine, but I might suggest using 8 x 3 hour blocks. You still need to sleep 3 and can still march 3 before you get exhausted, but it allows you some more options. Make it take a turn to set up camp (or risk being in a bad camp site with no tent, fire, defensive lines etc). People who take a watch to keep watch for dangers also still need three watches for sleep unless you want them tired, etc.
This means you CAN force march for additional hexes of movement, but you now face greater danger. How important is it to make it to the city walls by nightfall? Is it worth being exhausted the next day by a forced march? What happens if you run into an encounter than delays you by a hex and now you are exhausted in the wilderness?
8 x 3 gives just enough rope for parties to hang themselves with greed and recklessness.