Merge/concatenate/append spreadsheet documents, not spreadsheet contents

There is much information here and on the web about merging/combining the data of several spreadsheets into a single one. I want to do something much simpler but caot find how.

I have several files, say A, B, C, each containing a single Sheet1. I want to group the sheets into a single document, say D which will eventually contain Sheet1_1, Sheet1_2, Sheet1_3. This can be done "by hand", one at a time, using the Copy/move spreadsheet entry in the context menu of the spreadsheet tab. But I have around 40 files, so how can I automate this? I'll accept any method (shell script, document macro, or whatever).