I really think you can't have too many books. Some of these are hard to find, or are available only in Scandinavian countries, but I like collecting books like I enjoy collecting glass. I really feel you cannot look at too many photographs, or learn too much about the companies that made this glass and the circumstances under which it was designed, made and sold. And I think the Pina books are very useful, particularly if you supplement them with others. You really can't cpunt on any of them for pricing.
There are some good books that I have enjoyed and found useful (its true you only have to identify one unusual and valuable piece to make the books seem pretty cheap):
—The Best of Modern Swedish Art Glass: Orrefors and Kosta 1930-1970, Mark D. Friedman
—Modern Glass 1890-2000, The Danish Museum of Decorative Art
—Alvar and Aino Aalto as Glass Designers, published by the Iittala Museum
—Make Glass Not War, The Finnish Glass Museum, 1992
—Timo Sarpaneva, Collection, Designmuseo, Helsinki (best overall Sarpaneva reference)
—Timo Sarpaneva: A Retrospective, catalog for a show at the Helsinki City Art Museum (also traveled to the US) Great photos of high-end Sarpaneva
—Scandinavian Ceramic and Glass, 1940's to 1980's, George Fischler & Barret Gould
—20th Century Glass, Judith Miller, Collector's Guides (marginal, perhaps, but interesting)
There is also a great book on Kaj Franck that I saw in Stockholm, but sadly did not buy because it was so heavy and I was already loaded down and had to fly back to the States.
The 125 years of Iittala book is superb as well, a great overall reference.
Also, glass is often featured in general books about Scandinavian design:
—Finnish Modern Design: Utopan Ideals and Everyday Realities, 1930-1997, Bard Graduate Center for Studies in the Decorative Arts, Yale University Press
—A Treasury of Scandinavian Design; The standard Authority on Scandinavian-designed furniture, glass, ceramics and metal, Eric Zahle, Director of the Museum of Industrial Art, Copenhagen, 1961
—Scandinavian Design, Eileen Harrison Beer
—Scandinavian Design, Charlotte and Peter Fiell, Taschen
—Scandinavian Modern, Magnus Englund and Chrystina Schmidt