"Book paragraphs" vs "book snippets"

For a general term that describes each of those "snippets," I'd propose Layout Elements.

Layout elements serve to either provide information to readers or to attract their attention.

That term is widely used in webpage design, so, when you Google the term, you'll see a preponderance of results related to graphical web design. However, the term "layout" is borrowed from the early days of printing. This website does a good job of defining layout elements from the printed page perspective, and gives many examples as well. (I'll offer this one screen shot, to appease the link rot police, but this is just a small sample of what's in the entire guideline):

I think that they best be called Snippets. From Wikipedia:

Snippet is a programming term for a small region of re-usable source code, machine code or text. Ordinarily, these are formally-defined operative units to incorporate into larger programming modules. Snippets are often used to clarify the meaning of an otherwise "cluttered" function, or to minimize the use of repeated code that is common to other functions.

Since these sections may be repeated a few times, I may call them Siddur Snippets. Snippet will be describe granular text in the book, that may be repeated in somewhere else of the book.

Though probably not the proper linguistic term (if such exists), I would refer to them as blocks, indicating that they are self-contained elements on the page.

A paragraph can easily be considered a block of text, but so can a title, subtitle, etc.

I disagree with the use of "snippet" as that strongly implies a certain amount of reusability or recomposition of the elements, and I'm not getting that from your question at all.