Multi-volume titles
There are two types of multi-volume work. Either each volume has its own ISBN, or there is a single ISBN and the printed book is split into volumes for reasons of length.
In the case where each volume in a multi-volume work has its own ISBN, capture each volume in its own XML file, using book as the root element for each volume. Each book within a multi-volume work will therefore have different ID and DOI values.
In the case where a book is split into volumes for reasons of length, and each volume uses the same ISBN, same front and end matter, capture the entire book in a single XML file with a single copy of the front and end matter.