Here is the full list of all 20 Little Mouse's Encyclopedia achievements worth 1,000 gamerscore.
Go to all locations.
Explore all tabs in all locations.
Explore all tabs in the garden.
Explore all vegetables in the garden.
Explore all tabs in the pond.
Explore all tree tabs in the forest.
Explore all tabs in the forest.
Use the forest elevator.
Explore all tabs around the burrow.
Take a boat ride.
Go to garden.
Go to a pond.
Let all birds in the forest sing.
Wake up all sleeping forest animals.
Use index to get into a specific location.
Go to forest.
Awake all the insect in the grass in the first chapter.
Use the lens.
Finish the tutorial.
Go underground.