What does "I Know This Much Is True" mean?

Question

There is a novel titled "I Know This Much Is True", and I find the title puzzling.

I know this much is true.

At first, I parsed it as

I know this much it's true.

where this much shows the degree of (un)certainty, serving as an adverbial phrase. As in, "I definitely/certainly know it's true" with the order changed a bit.

Then I noticed it's "is true", not "it's true", which leaves me with this sentence structure.

I know (that) this much (=something) is true (=something).

What could this much refer to? Is it something contextual and I need to read up on the novel?

I am thinking of

I know that (giving/receiving/having) this much (love/care/something uncountable) is possible/true.

but there is no reasonable grounds for that...

Answer 1

The phrase “this much” is an idiom when used in a phrase like “I know this much is true”, so there isn’t a literal parsing that captures the meaning. The definition in the Free Dictionary explains it as:

phrase used to indicate a minimal but definite piece of information.

We can use it in many different ways—“know this much” and “say this much” are fairly common. Here is another example:

After reading ELL, this much was obvious: English is a difficult language to learn.

Answer 2

If you're sharing a cake with someone and you say "I would like this much", you're telling them how much of the cake you would like. If you say "This much is true", then you're saying that some amount of something is true, probably there's a number of things that might be true, and you're sure some of them are.

Hence, "I know this much is true" is a partial sentence meaning that of all the things under discussion, you know some part of them to be true. It's a partial fragment of a sentence because it's being used poetically for emphasis in a book title.