This is a good general introduction. The scientific references (bibilography) are useful. He also mentions the important role of historical Alaska Native tissue samples from 1918.
Nicholls H (2006) Pandemic Influenza: The Inside Story. PLoS Biol 4(2): e50
The origin of the 1918 pandemic strain, by contrast, has been harder to crack. Nearly a decade ago, Taubenberger and his colleagues made a real breakthrough; they found isolates of the 1918 pandemic virus in the formalin-fixed, paraffin-embedded lungs of an American serviceman  (Figure 4). They subsequently retrieved further samples of this deadly virus from a second soldier and also from a flu victim exhumed from a frosty mass grave in Alaska. Since then, they carefully sequenced one gene after another until they completed the task last year .
Box 2. Outbreaks in History
In the past century, there were three influenza pandemics. The “Spanish influenza” of 1918 caused by the H1N1 subtype is estimated to have hit nearly a third of the world’s population (Figure 3A). Conditions at the end of World War I may have contributed to the mortality; in just one year, it killed more than 40 million people (Figure 3B). The “Asian influenza” of 1957 resulted from a reassortment event, generating a new subtype: H2N2. There was little or no pre-existing immunity to this reassorted virus, and it is believed to have caused more than 2 million deaths. Eleven years later, further reassortment gave the prevailing human influenza a new surface protein, resulting in H3N2. This subtype killed about a million people and is currently the dominant subtype of human influenza.