It can be studied very well through his words?
No. Are is never can in English. The author is saying, “It is best studied through his words.”
the character and ideas of a statesman are best studied through his own words.
Another way of saying this is:
If you want to understand the character and ideas of a statesman, it is best to examine what they themselves have to say.
Or in more simple English:
To understand a politician’s beliefs, you should listen to what the politician says about these beliefs (instead of what others have to say about them).