Translation:There is one book on the bookshelf.
Maybe at first they introduce in hiragana and then replace it with kanji
So I'm curious about the structure. Litterally translated it is "book bookshelf on one is." So grammatically the words "on" and "one" are describing the book actions and features (makes sense since the が expresses that is the subject that will do stuff). So how would you say something like there is 3 books on 2 bookshelves. Where the bookshelves also have adjectives? Where would the bookshelves' adjectives fit?
が places more emphasis on the subject. That means it's often used in situations where that subject is really noteworthy, unexpected or grammatically necessary (like after an interogative pronoun and its response). は is a general topic marker, and used in cases where the subject has already been established, or when there's an implied contrast between something.
Take 本が本だなに一さつあります。In this case 本 is stressed, either because someone asked what's on the bookshelf (なにがほんだなにありますか), or because you wanted to draw attention to this fact for some reason. Had it been 本は本だなに一さつあります, it is a subject that could be left out (e.g. after someone asked if you have a book anywhere), or that's used to points out that the thing you have one of is a book, not a magazine or comic.
In case of the latter, you could add が after 一さつ if the quantity of books was the target of a question or needs emphasis.
They are not required to stay together in Japanese. Contextually it's just about what you want the listener to know first, and in this sentence they choose to add the location: 'book(s)' / on the bookshelf / one / there is. Translating 一冊 (literally "one volume") as "alone" is incorrect.
It's a bit odd (compared to what we've done before, at least) to have a non-topic subject before the place, but if the number had been supposed to go with the bookshelf I believe it would have been placed in front of it (so that it would be included before the に): 本が一さつの本だなにあります. Instead, it is connected to あります, which links it to the subject (even though that isn't anywhere close).