I’m assuming by “RPG book” you mean a prewritten adventure or setting? The best way to convert those is by matching the feeling of the mechanic rather than trying to do a 1 for 1 conversion, it takes a little longer to start with but you won’t end up with an unplayable mess so it’s worth it.
For converting NPCs, make use of the NPC statistics table in the Running the Game chapter. Think about how much of a challenge they’re supposed to be and in what ways they’re supposed to challenge the player; if they’re supposed to be physically tough give them high HP and defences, if they’re meant to deal damage they should have more advantage on attacks, if they have special effects try and match them up to banes and boons, if the effect doesn’t seem doable with banes or boons consider if you can copy it word-for-word (e.g. burrowing).
For things like traps, diseases or other effects, Extraordinary Items probably have you covered (especially if you don’t assume that it has to be a physical item). Traps make attack rolls versus defences and probably deal lethal damage or apply banes with a set duration, if a player wants to make a save then that’s a Defend Interrupt (covered in the Combat Chapter).
If you come across some problems while attempting this, please come back and explain the specific issue. It’s going to be a lot easier for people here to give suggestions for your particular case than it is to give general advice that suits all situations, because that kind of advice doesn’t really exist.
If I’m wrong in my assumption about what the book is and there’s actually some mechanics you want to copy for their own sake, I’m afraid you’ll really struggle to do that without making the play experience worse. OL was built around not having those things and so trying to fit them in is going to be awkward and difficult.
If you want those mechanics in your game, why not play D&D instead? If you want to play OL, what do you feel is missing that you need to include those things? If neither D&D or OL are a good fit for what you want, I’m afraid it would probably be easier to build an entirely new RPG system from scratch than try and combine them.