To combine multiple Word documents, use Insert > Text from File.
You can use Word's If...Then...Else to make a mail merge field have a condition. If one thing occurs, do this. If it doesn't occur, do something else.
An index is a great way to lists the terms and topics that are discussed in a document, along with the pages that they appear on.
By default, Microsoft Word performs a mail merge and One to One, but you can use Graham Mayor's add-in to make Word perform a Many to One mail merge.
To perform a letter mail merge in Microsoft Word you need a data source. Excel is a great data source. This covers setting up your letters - Chris Menard