You can watch a video that demonstrates what you can do with MystCraft here.
Introduction to Mystcraft
Mystcraft is a relatively simple core mod of Tekkit Lite, adding only a few items, where you can travel to different dimensions through books that you write. It was modelled after the award-winning puzzle game Myst.
Mystcraft adds the ability to create different books that will take you to new dimensions called ages. To start, you'll be sent to worlds with random configurations. But in time you can set what kind of attributes you want for the age. Not all ages are good homes, however, as some are unstable which causes the world to have negative side-effects ranging from simple intolerance to the sun to raining meteors. Ages become unstable if they are not properly written or are written to offer overbearing benefits.