Is the term 'By far' an idiomatic term for 'Very much'?
For example, can I say: 'I like this book by far'?
Does it mean 'I like this book very much'?
Are 'By far' and 'Very much' interchangeable in all contexts?
I guess 'By far' should be more informal than 'Very much'. And maybe I'm wrong.

Question

Answer

"By far" means "by a large margin" and is usually used in comparisons. For example, "Charles is by far the coolest person I know" means Charles is much cooler than even the second coolest person - there is little doubt.

So, it does not make sense to say "I like this book by far" because there is no comparative quality and "by far" does not mean "very much".

Another example of it's use might be: "John did by far better than James: he got twice as many points as him."

Hope this helps!

Answer

No. "By far" intensifies a superlative.

If it was 18° on Monday, 20° on Tuesday, and 17° on Wednesday, then Tuesday was the hottest day.

If it was 18° on Monday, 31° on Tuesday, and 17° on Wednesday, then Tuesday was the hottest day by far.

Answer

You can say: 
So far, I like this book. 
So far, this book is good/ bad/interesting etc.

I interpret "so far" as up to this point in time. 
"Up to this point, I like this book."