A family of Microsoft word processing software products for creating web, email, and print documents.
I am sure that there is no "best" book or series of books. Everything about Word would require a multi-volume series of books. Likewise with Excel. These programs have been incredibly complex for many years.
In the way they work, Word and Excel in Office 365 are not greatly different from Office 2003 even though the interface has changed and MS has added lots of bells and whistles to attract the corporate accounts.
Recently, I have very much liked the Word in Depth series by Faithe Wempen. The last one in that series was for 2016.
This forum is an excellent resource.
Another would be the Mail Merge forum.
I use mail merge on a regular basis and have no problem with a primary merge document forgetting the data source.
However, I usually use a macro that converts a regular word document to a primary merge document and attaches my principal data source. This is because I use templates for the primary and usually make changes to a new document before performing the merge rather than always using the same merge. When I reopen one of these documents, the data source is remembered.