Learn Polish: Książka zostanie opublikowana przez to nowe wydawnictwo. - The book will be published by this new publishing house.

In Polish passive voice, we use zostać (here in the future tense as zostanie) with perfective verbs to focus on the completed result. If we used będzie, we would need the imperfective participle publikowana to describe the ongoing process of being published.

How does przez work to express who does the action in this sentence?

To show who or what is performing the action in a passive sentence, Polish uses the preposition przez followed by the Accusative case. Here, przez points to the agent doing the publishing, which is the publishing house.

If przez takes the Accusative case, why does to nowe wydawnictwo look like it is in the Nominative case?

The noun wydawnictwo is neuter. For all neuter nouns, as well as inanimate masculine nouns, the Accusative case form is exactly the same as the Nominative case form.

Why does the participle opublikowana end in -a?

Passive participles act like adjectives and must agree with the subject of the sentence. Since the subject książka is a feminine noun, the participle takes the feminine singular ending -a.