The action of truth is just truth itself, though it's also facts and reality as well. Really, it's the idea of "truth" in a way.
This is a jukugo word, which usually means on'yomi readings from the kanji. If you know the readings of your kanji you'll know how to read this as well.
All of the above is true.
This book has many facts about mathematics.
It's a fact that all Japanese people know, but crabs in Hokkaido are delicious.