Let's Face it, books take forever to make. Especially Codex and Tomes! - After weeks or months of cooking manuscripts you don't want to throw away 50% of the books applied on a explorer build you regret, and you definitely don't want to waste 1000 gems to get a refund for misplaced books.
Here are a couple of versatile builds for you to consider:
I found this to be the most versatile and best build for your lucky explorers. You have two options here - both options give weight to artefact searches. You will always have that as an option. The first build favors adventure searches, with the last update removing certain adventures from the adventure shop you might be more interested having an explorer that searches for adventures.
The second build is for treasure searches, well more importantly gives weight to artefact search, however when you are around for events and need to find those event resources, this build favors those very long treasure searches and reduces the duration of the search as much as possible.
Each build has versatility so you do not have to beat yourself up in case you made a bad choice with 1 build or another. Personally, I applied the 2nd build for my explorers. What i give up is a small '10% reduction in adventure search costs', and the ability to get lucky with '20% chance at 2 adventures'. Which is fine for me because I only intend to run artefact searches, and strictly very long treasure searches when events are active.
In case I need to do an adventure search out of the blue some day, my adventure search options will still be good, as the cost is still cheap to execute those searches.
If your a fan of artefact search and adventure search, the 1st build is the one for you. You will get '20% chance at 2x adventures', but your artefact search will be under 7 hours. By switching 1 codex from the build I have provided, you can probably choose '30% chance at 2x adventures' as well and lose '5% time reduction' in your artefact search which will go up to 7 hours. You have to be your own judge and decide what you will do more. You will lose a small 3% of event resources (probably 1 event resource in most cases) because you chose to maximize the reduction of the adventure search costs.
These are simple outlines to maximize the versatility of your explorers. They will always be good at artefact search, and either adventure search or very long treasure search.
I provided the search timings and adventure search costs for you so you don't need to waste books experimenting.
Good luck to you all in choosing your own builds and Happy Settling.