First the two books which were my main source for the uniforms (such as there were) and flags of the troops. That latter part can be taken literally, as many of the flags on the miniature units are scans from these books.
* _The Great Northern War 1700 - 1721, Colours and Uniforms_. Lars-Eric Höglund and Åke Sallnäs, Acedia Press
* _The Great Northern War 1700 - 1721, II. Sweden's allies and enemies, Colours and Uniforms_. Lars-Eric Höglund, Åke Sallnäs and Alexander Bespalow, Acedia Press
Both these books can be bought from Pete Berry at [Baccus Miniatures](http://www.baccus6mm.com) (check under Products/Publications).
Next are two books on the history of the Great Northern War and more specifically the Russian campaign and the battle of Poltava:
* _The battle that shook Europe. Poltava and the birth of the Russian empire_. Peter Englund, I.B. Tauris
* _The dawn of the Tsarist empire. Poltava and the Russian Campaigns of 1708-1709_. Nicholas A. Dorrel, Partizan Press
Both are good books, the second going more into the military detail of the campaign and providing good scenario material. The OOB of the units on our Poltava game was based on this latter book. The first should be generally available, the second can be ordered from [Caliver Books](http://www.caliverbooks.com).
And finally there's three Ospreys that deal directly with the campaign:
* [Campaign 34 - Poltava 1709](http://www.ospreypublishing.com/store/Poltava-1709_9781855324169)
* MAA 260 - Peter the Great's Army 1: Infantry
* MAA 264 - Peter the Great's Army 2: Cavalry
Unfortunately, the book on the Russian infantry seems to be perpetually out of stock. Also, there are no Ospreys on the Swedish army of the period (apart from some plates in the Campaign book on Poltava).
On the web
Dan Schorr's [Northern Wars](http://www.northernwars.com/) page had some great information on various aspects of the Great Northern War, but the page has gone offline recently. I mention it in the hope that it will come back someday.
Wikipedia has a good page on [the Great Northern War](http://en.wikipedia.org/wiki/Great_Northern_War) and on the [battle of Poltava itself](http://en.wikipedia.org/wiki/Battle_of_Poltava).
Finally, there is good content relating to the Great Northern War at [Wikimedia Commons](http://commons.wikimedia.org/wiki/Great_Northern_War), much of which is in the public domain.
The above list should get any gamer interested in the Great Northern war started in this fascinating bit of history.
*Update 13/11/2010*: the Wyre Forest Games Club has a number of very good pages on the Great Northern War. Start [on this page](http://www.wfgamers.org.uk/resources/C18/gnw.htm) and then explore using the menu at the top of the page.