The change log gives a fairly detailed description of the new features in each release.
KERBAL SPACE PROGRAM FORUM HOW TO
The FAQ explain how to install, how to report bugs and documents the known issues and limitations. We also have a guide explaining how to use the support for rendezvous. To go to the Mun with Principia in an energy-efficient manner. In particular, you should learn about the plotting frame and flight planning. Therefore, before using the mod we recommend that you read the concepts document which explains the most important parts of Principia. N-body gravitation is more complex than the toy physics of the stock game.
This makes it possible to implement missions that are more complex and more realistic than in the stock game, especially if used in conjunction with a mod like RealSolarSystem which has real-life celestials. Instead of being within the sphere of influence of a single celestial body at any point in time, your vessels are influenced by all the celestials. Principia is a mod for Kerbal Space Program (KSP) which implements N-body and extended body gravitation. ابن الهيثم, the May version of Principia, is available, with support with displaying equipotentials and navigating to the Lagrange points.
If you downloaded ابن الهيثم before May 20, 14:08 UTC, you probably have a buggy version (see #3653).