The most important book for a collector is The Identification of English Pressed Glass by Jenny Thompson who not only covers all of the main manufacturers but also details the registration marks and lozenges and trade marks.  The Millers guide on Popular Glass of the 19th and 20th Century is also very useful because it details all the different types of pressed glass produced and their prices.

Unfortunately the rest of the books are very hard to find and when they do come onto the market, they go for very high prices, which leads to the question, do you spend £50+ on a book, or buy a quality piece of glass?!  At the Spring Glass Fair, I was talking to a book seller who told me that a Tyne and Wear publication on Jobling which was initially sold for £4 has been sold for nearly £60.

Books both in print and out of print

Popular Glass of the 19th and 20th Century - Raymond Notley (2000, Millers Guide) still in print

The Identification of English Pressed Glass by Jenny Thompson  + Supplement (1993) This is a great book for anyone who is interested in glass. still in print

Pressed Flint Glass - Raymond Notley (1997, Shire) still in print

Sowerby Gateshead Glass - Simon Cottle (1986, Tyne and Wear Museum Service) out of print

The Peacock and the Lions, the story of pressed glass of the North East of England - Sheilagh Murray (1982, Oriel Press) out of print

English 19th Century Press Moulded Glass - Colin Lattimore (1979, Barrie and Jenkins) out of print

Davidson's Glass, From Gateshead To The World - Souvenir from the Davidson Exhibition at Shipley Art Gallery out of print

British Glass Between the Wars - Roger Dodsworth (1987, Dudley Leisure Services) out of print

One of the best ways of finding out more about pressed glass is to go to the Library at the Victoria and Albert Museum in London and look at copies of the Pottery and Glass Gazette where the manufacturers advertised their new suites of glass.

Websites

Cloud Glass.com
Chris and Val Stewart have researched so much stuff about Cloud Glass and the Davidson Factory.  It is a fantastic site and even has an up to date list of what cloud pieces have fetched on eBay, well worth a visit.

Glass Museum Online
Here is a brilliant site by Angela Bowey.  There are articles on every type of glass that you can imagine, including an interview with Sheila Murray's daughter about her collection of pressed glass. 

Glass Encyclopaedia
This is Angela's other site, which is exactly what it says, an encyclopaedia on glass.  Like the Glass Museum, it is very well researched  and incredibly useful.

1st Glass Man
This is Tony Hayter's site, who sells glass on eBay, it also has a lot of articles, photos and the brilliant Lozenge Translator!

Pressglas-Korrespondenz
This is a German site, but even if you don't speak the language, it has a lot of interesting catalogues and photos.

Glass Message Board
Everyone is very friendly here and are incredibly knowledgeable about pressed glass, so if you have any questions, or just want a chat, then this is the place to go.

The Glass Man
This is a dealer site, which despite being a little dear, has many articles and very comprehensive history of all of the main manufacturers.

Great Glass
Here is another dealer site, which also has a lot of information about pressed glass.

Glass Makers of Knottingley
This is part of the Knottingley and Ferrybridge site and has a lot of information about this history of Knottingley glass makers, who include Bagley.

The Flower Frog Gazette
This site features information about Flower Frogs including those made by Sowerby, Bagley and Walther.

The Victoria and Albert Museum
Visiting the Museum's glass rooms is an amazing experience, but sometimes it is difficult to visit, so why not let the Museum come to you!  Buried within the site is a search facility where you can search their photo catalogue, it takes a while, but you can see many of their exhibits.  Try a few search terms and see what you come up with!

Tyne and Wear Museums Website
This search facility can help you navigate the Tyne and Wear Museums website.  There are several photos of a glass press and many photos of the Sowerby range and even a portrait of George Davidson, a link of which can be found in the Davidson section.

National Glass Collectors Fair
This Fair is held twice a year in the Midlands and is absolutely huge.  It has every type of glass from Sowerby to Whitefriars and you can often find a bargain or three there!

Take Five Fairs
There is a glass Fair in Woking run once a year, Art Deco Fairs five times a year as well as regular antique fairs the rest of the time.  There are loads of brilliant stalls, with very friendly people - even in the Art Deco Fairs you can find Victorian and Edwardian glass and even some Whitefriars, so it is always worth a visit!

Antiques Trade Gazette
This is a website where you can find details on most of the auctions in the UK, some even have online catalogues.  It takes a bit of navigating, but you can find some gems with a bit of effort.