Back when I was a graduate student, there was one undergraduate DM who gained the nickname 'Neon Bruce' because if you ventured into an area of his dungeon that he didn't want you to visit yet, you encountered a large neon sign that said 'No Entry'!
Perhaps this work would have enabled him to cope with straying characters, as it is intended to empower the GM to create encounters on the fly, selecting and slotting in appropriate monsters as and when you need them. This, the first of (we hope) many products, deals with providing you with stacks of ready-made EL1 encounters worth 400 XP to those who defeat them successfully.
It is based around three big tables, on which you can roll percentage dice (or just pick an encounter if you prefer to do so). One is for dungeon areas, one for cavern areas and one for the classic 'wandering monster.' The dungeon area ones are most suitable if you are delving a constructed dungeon, and the cavern ones for more natural underground places (the two, of course, may intersect). Wandering monsters, by their very nature, can turn up just about anywhere, provided you can reason out a route for them to get there.
Some of the encounters involve regular monsters from the Pathfinder RPG 'Bestiary' books. Some involve variants which have the advanced, giant or young templates applied to them, and some are unique monsters whose details are given here.
So, let's get the bones out and see what's here... OK, an 89 gives me a wererat! But there's more, I'm provided with his name and a bit of background as to why he's here in the dungeon - apparently he's ugly even as wererats go so was happy to accept a job from whoever-it-is that constructed this dungeon to keep this area clear of intruders. By and large he does a good job, but he's not above stopping for a chat! So in a couple of short sentences the encounter is transformed from 'Here is a monster to brawl with' to a whole opportunity to make a true encounter of it. Are the characters perpared to chat or will they just lay into him? What will be the outcome of a conversation? Quite delightful, isn't it. Because this is a named specific wererat, not just any old wererat, he's got a complete stat block provided on the next page so we have all the details necessary to play him - in conversation or in combat, as the need arises.
Even when your roll selects a 'standard' by-the-book monster, there are still additional snippets to make the encounter more interesting. In the caverns table, for example, your roll might result in two mites and a giant cockroach... only the cockroach is the mites' pet and they are trying to teach it to jump through a hoop. Poor 'roach isn't very good at that, but it's a loyal pet and will attack anyone who goes for the mites!
The whole book is full of stuff like this: pure genius. Worth using when planning dungeons, never mind when caught out by pesky players wandering off where you hadn't expected them to go!