Learn Hardcore German: Nur ein Buch ist interessant.

Because it’s the subject of the main clause. In German, the subject of a sentence takes the nominative case. Here ein Buch performs the action of sein (“to be”), linking it to the predicate adjective interessant.

Why is nur placed before ein Buch and what exactly does it modify?

Nur is an adverb that limits or restricts whatever follows. By placing it before ein Buch, you’re saying “only one book” (as opposed to two, three, or none). It directly modifies the noun phrase ein Buch.

Could I say Ein Buch ist nur interessant? Does it mean the same?

No. Ein Buch ist nur interessant shifts the emphasis to the adjective: “A book is only interesting” (implying it’s not useful, important, etc.). It doesn’t convey “only one book among many is interesting.”

Why doesn’t interessant receive an ending like interessantes?

Because interessant here is a predicative adjective following the linking verb sein. Predicative adjectives in German remain uninflected, so they don’t get adjective endings.

What gender is Buch and how does that affect the article?

Buch is a neuter noun. In the nominative singular, neuter nouns take ein as their indefinite article, hence ein Buch.

Why isn’t there any comma in this sentence?

It’s a single, simple main clause without subordinate clauses, lists, or parenthetical elements. German only requires commas for certain structures (e.g., clauses, enumerations), none of which appear here.

How would I say “Only the book is interesting” instead of “only a book”?

Switch to the definite article:
Nur das Buch ist interessant.

How can I emphasize “only one single book” is interesting?

You can add einzig(es) to stress singularity:
Nur ein einziges Buch ist interessant.