Subordinate clauses in sentences

Your use of the subordinate constructions is correct in both phrases.

Strictly speaking, only "How they love each other" in the second sentence is a "subordinate clause" in the sense of being a subordinated proposition having a conjugated verb. It works as the subject of the second sentence.

Within the clause:

• "They" is the subject
• "How ... they love each other" is the predicate

By contrast, "how to control your emotion in difficult situations" is a subordinate, substantive construction, but it is not a "clause" as it has no conjugated verb, and it has no identifiable subject or predicate.

It works as the "term" part of a "circumstantial complement" that indicates what the book is about (subject/theme/reference), subordinated to the main sentence via de preposition "about".

In relation to your question about subordinate clauses (or constructions) being nominal (substantive) in nature, or equivalent to nouns, the answer is yes. That is a perfectly normal feature of the language.

However, notice that only the second of your clauses, "How they love each other" is purely nominal/substantive, as it is equivalent to a subject.

Your full second subordinate construction is "about how to control your emotions in difficult situations", and that is not exactly nominal, although the term part of it, "how to control your emotions in difficult situation", is.

Punctuation

One of the function of puctuation (specifically, commas) is to denote that the expected word order of a sentence has been changed somehow.

Compare:

"Murder", she wrote.

She wrote "murder".

In principle, both your sentences respect the overall expected order of the "parts of a sentence" in English: Subject - Verb - Object/predicative.

So neither commas nor other punctuation are necessary.