What is truth?
Truth is a very relative term. There is no strict definition about what true actually is. Different cultures, time periods, and religion all affect what people accept as the truth. New discoveries and new technology can cause people to change their viewpoint about what is "true." It is something that cannot be proven, it can only be disproven, and people will accept somethng as true until it is disproven.
No specific person says what is and what is not the truth. Truth is just something that is accepted by society as a whole because no one has any other explanation for something.
I don't necessarily agree with what the book says about truth. The book says that truth is finding the best possible solution for something. But what is the best possible solution? This solution could be different for everyone. Even if you go with what is the best solution for the majority of society, there are still many people that will have a different opinion.