There is little written on the Metal Ware Cooperation "Empire"
The only one well known and best book on the subject is written by Don Stilson.
But that book was for many years not available anymore, unless you could lay your hands on a secondhand one.
And since Don Stilson is deceased, the chance of seeing a reprint was very slim.
Buy now, the daughter Deborah Castle, issued a reprint of her fathers book, that you can buy for 25$.
And I think that's a fair prise.
You can mail her at castleda@prodigy.net, and place your order direct with her.
So, for those among you who where longing for this book for so long, this is your chance.
There is also a 1935 digitalized free full catalog on the net:
https://archive.org/details/EmpireEducationalToys1935/mode/2upHave a look at it it's super nice done
Cheers,
Danny