I apologize for the delay in adding this:
Loetz Tango Glass
by Herman Spaink
This book, which has become widely used as a reference on ebay, and is also making it's way into other auction venues including brick and mortar establishments, is not worth the investment unless one is looking for a nice coffee table book for pictures alone.
The information contained in this book actually flies in the face of known and documented information. There are rampant and incorrect attributions of Tango pieces to Loetz and also Michael Powolony. It is well known that Powolony designed a hand full of Tango pieces for Loetz, yet this book incorrectly attributes many pieces to both Loetz and also to Powolony. Powolony designs are well documented and easily identifiable.
Additionally the book creates the attribution to a manufacturing company by the name of Loetz Kralik, implying that the companies are one in the same. This has contributed to the rampant attribution of many pieces of Tango like glass on ebay to both Loetz Kralik and also to Powolony, and cite the book as the attribution reference.
The glass pictured in the book is represented as being part of the collection in "Het Haags Glasmuseum. Den Haags ND" Many serious attempts at locating and verifying both the museum and the collection have failed.
If you must buy this book, buy it for pictures only, if what you would like is a nice selection of Tango Glass images. The photography is actually well done.
At the time of writing it appears that the website promoting this book has been taken down, as I was unable to locate it any longer.
For the sake of accuracy not take the text in this book seriously. It has done serious damage by helping to perpetuate the internet myth that all two colored (or close) Czech glass is Tango, that all Tango glass is Loetz (pr now Loetz Kralik), and that all Loetz (Loetz Kralik) Tango was designed by Powolony.