Since you state that you are using an alternate word processor (why the mystery here?), Pages cannot open any other document format than compatible Word (.docx, .doc), plain text (not markdown, not LaTeX), or Rich text (RTF) documents. If your multiple files are not in one of those formats, you will need to export or convert them first.
Pages has no means to automatically merge multiple documents, or link external documents into a single Pages document. That means that if your goal is one document, you will need to perform individual opening of those many documents and perform copy, paste, and reformatting into the target Pages document. Opening those documents in your alternate word-processing application and attempting to copy/paste may or may not go well.
The maximum file size allowed to publish to Apple Books is 2 GB. Otherwise, Apple does not mention the maximum document size. Pages will become slower as document size increases.
You might consider the paid (free trial) of Literature & Latte's Scrivener instead of Pages. It is in the Mac App Store.