You can find also a number of additional information on opc on this site. Yes, there are also books on opc.
One is only available in german and gives a short overview on opc from the viewpoint of vendors.
You will not find much about coding there.
Then there is a japanese one, that describes how to program opc apps with VB.
I have not seen this.
Then there is the book from softing on OPC, available in German and English. It is sold out. A second edition will be there in June.
If you need more information, feel free to contact me directly.