Fact or Opinion

Given that the passage is about music therapy, and the writer is the director of music therapy, we can say that his statements on that subject should be taken seriously.

The book contains techniques that some people may find useful.

We assume that there are “techniques” in the book – that much can be take as a fact.

The question is whether “some people may find some of those techniques useful” is a fact or not.

The statement “The book contains techniques that some people may find useful.” Addresses the entire population of the world.

The “some people” would only require there to be three people out of 7.5 billion who may find any one technique to be useful. Of that population, there is bound to be 3 people who “may” find a technique useful as we are including the stupid, the genius, the writer, the writer’s friends and the downright gullible. (It is possible that all the techniques are very good.)

The sentence is a fact. That the sentence expresses an uncertainty, i.e. “may find”, is irrelevant. A fact can be uncertain: “That wall might fall down” and "You might guess the number I am thinking of" are facts.

If the writer had said “The book contains techniques that some people will find useful.” That would be pretty close to a “fact” but just falls short as there is no certainty that it is true, yet it purports to be.