merge two files into one pdf book

You can use a Book file, which will let you export a PDF containing all the pages from one file then all the pages fron a second file, and so on. If you have a front and back cover in one file and the interior pages in another file this will not work.

I suggest something like Eugene’s solution. Place the InDesing file of the cover into black pages at the beginning and end of the main file. Use sections to preserve page numbering so page 1 of the book is still page 1, even if it is no longer the first page.

Start a new document with the size you want

File>Place to place the cover page - you can place the InDesign file

Then use File>Place again to insert all your other pages - you can tick Show Options and the select All - then place each page individually

Then place your back cover page

Then you can publish online.

As it's linked to the original InDesign files - once you update the content - open your digital edition and update all the links and you can then update the Published Online version.

If you have a lot of pages you can you import them in one go

https://creativepro.com/zanelli-releases-multipageimporter-for-importing-both-pdf-and-indd-files/