To view it all together, create a PDF file from the book file.
They are separate document, so you could open them all, but it's not the same as being in the same file.
If you have a reasonably fast computer, it might be better to keep it in one file. Many book people prefer to do that.
The whole point of a Book is to break the documents apart into easy-to-work-with chunks so you aren't flipping through the whole thing while you edit. You can, as Steve says, export to a single PDF, but there is no way to move from one included file to another inside the .indb without manually opening each of the files you want to visit.