Apart from general articles on cards in online encyclopedia and game portals, there are two main online information sources: collectors/researchers associations websites and collectors personal websites (some of the latter being experts themselves).
I found the following the more interesting:
• International Playing Card Society (http://i-p-c-s.org/) has very informative sections, like ‘History of cards‘ (a very good historical introduction), ‘FAQ‘ (comprehensive list of resources) and ‘Pattern sheets‘ (inventory of patterns with historical discussion).
• Andy’s Playing cards (http://a_pollett.tripod.com/cards.htm), by Andrew Pollett, presents an exhaustive historical synthesis and detailed information about worldwide local patterns. One of the rare occidental sites to look deep into japanese cards. In english and italian. Sadly the site isn't maintained any more, many pages are missing.
• The world of Playing Cards (http://www.wopc.co.uk/), by Simon Wintle, offers a bunch of short thematic articles about patterns, makers and historical developments on cards of the world.
• Alta Carta (http://www.altacarta.com/english/goal.html) is an exchange platform for collectors. It has many useful articles. See sections ‘Research‘ and ‘Collecting‘, available in several languages.
• English site Plainbacks.com (http://www.plainbacks.com) is dedicated to english pattern cards. It illustrates some of the most ancient decks of this type.
• Amongst collectors and experts, Peter Endebrock (http://www.endebrock.de/), specialist of the Hanover area cards, gives essential data for the question of dating cards, using tax stamps and similar marks (see ‘Tax stamps‘). His collection is also worth browsing. In english and german.
• Vögel Karten (http://www.froja.de), personal website of Heiko Buchali, contains many illustrations of his collection, though sometimes lacking detailed reference. In german.
• I also often refer to World Web Playing Cards Museum, the colossal endeavour by russian collector Alexander Sukhorukov to document each and every deck published in the world(!!): (http://a.trionfi.eu/WWPCM/). The quickest to identify a deck (but sometimes approximate). Mainly in (strange) english.
• Pagat.com (http://www.pagat.com) is also worth mentionning. This site references games (names, history and rules) of the world.
Museums, private collections
Being for a while in Fukuoka and living in Paris, I had the chance to visit two of the few museum in the world dedicated to playing cards (respectively the Miike Playing-Card Memorial Museum in Omuta and the Musée Français de la Carte à jouer in Issy-les-Moulineaux). Admiring part of their collection, exploring the written documentation played an important part in the decision to start my own collection.
I.P.C.S. website gives a comprehensive list of playing card specialised museums or institutions whose collection has interesting decks.
I regularly use the following reference books, to check dates and description data. (The bracketted name is the abreviation used in this site to refer to the corresponding document.)
• Felix-Alfaro FOURNIER, Playing Cards – General History From Their Creation to the Present Day, H. Fournier, S.A., 1982. [Text in english]. A presentation of the very important collection held at Fournier Museum (Vitoria-Gasteiz, Spain), established by the grandson of the Fournier company founder, later augmented by acquiring the collection of english manufacturer De La Rue. This book describes a selection of about 2000 decks. Descriptions are not without mistakes, as noted by Ken Lodge in his blog, on this page.
• Franz BRAUN, Schriftenreihe „Spielkarten“ – Band 3, Köln und seine Spielkarten-Hersteller, Köln, 1990. [Text in german]. History of manufacturers on the german city of Cologne, with many illustrated editions.
• Franz BRAUN, Schriftenreihe „Spielkarten“ – Band 4, Die Spielkartenfabrik B. Dondorf, Köln, 1991. [Text in german]. Monograph of production from german manufacturer B. Dondorf.
• Franz BRAUN, Schriftenreihe „Spielkarten“ – Band 2, Die Spielkartenf in der DDR – 2 Auflage, Köln, 2000. [Text in german]. Reference book for cards edited in East Germany from 1945 to 1990 (mainly VEB Altenburg Spielkartenfabrik).
• Franz BRAUN, Antal JÁNOSKA, Schriftenreihe „Spielkarten“ – Band 5, Die Spielkartenfabrik Piatnik in Budapest und ihre Nachfolger, Köln, 1992. [Text in german]. Monograph of production from austrian manufacturer F. Piatnik and its subsidiaries in Hungary.
• Tom & Judy DAWSON, The Hochman encyclopedia of American playing cards, Stamford, 2000. [Text in english]. Reference book for american decks, a compilation of the volumes formerly published by Gene Hochman between 1976 and 1981.
• Detlef HOFFMANN, Margot DIETRICH, Die Dondorf’schen Luxus-Spielkarten, Harenberg, Dortmund, 1981. [Text in german]. This book is the result of an in-depth study of german manufacturer B. Dondorf factory catalog. Each sample is illustrated and commented.
• Ulrich KNÜPFER, Schriftenreihe „Spielkarten“ – Band 16, Von Baltikum zum Balkan - Spielkarten aus verschiedenen Ländern Europas, Köln, 2004. [Text in german]. Reference book for cards edited in various countries in central Europa and Baltic states.
• Sylvia MANN, All cards on the table, Leinfelden-Echterdingen, 1990. [2 volumes, text in english and german]. Sylvia Mann, who owns a very important collection of cards, was the first president of I.P.C.S., in recognition of her huge categorization entreprise of common playing cards from all over the world. All cards on the table presents this work and is also the catalog for the exhibition of her collection held in Leinfelden-Echterdingen (Germany) in 1990-91. Each pattern is illustrated by one or more samples from the collection, together with descriptive and reference notes.
• Martin SHAW, Paul SYMONS, Playing cards from the factory C.L. Wüst, Frankfurt, Germany (1811-1927), Turnhout, 2005. [Text in english]. Catalog of the exhibition on german manufacturer C.L. Wüst, held in 2005 at Nationaal Museum van de Speelkaart (Turnhout, Belgium), curated by the authors.
• Dieter SCHROERER-LLISCH, Schriftenreihe „Spielkarten“ – Band 7, Die Bielefelder Spielkarten-fabrik, Köln, 1998. [Text in german]. Monograph of production from german manufacturer Bielefelder Spielkarten-fabrik.
If you have additional documentation, you can notify it to me: firstname.lastname@example.org.