Alfred W. Crosby, America's Forgotten Pandemic: The Influenza of 1918
John M. Barry, The Great Influenza: The Story of the Deadliest Pandemic in History
Laura Spinny, Pale Rider: The Spanish Flu of 1918 and How it Changed the World.
All very good, and quite different in how they deal with the topic. Crosby and Barry are mostly concerned with the US and with American troops serving in Europe, while Spinney (who is British) ranges more widely.
Current estimates are that 50 to 100 million people died around the world. The figure sometimes seen for the US, 675,000 deaths, does not include those who died after the middle of 1919. A final wave in early 1920 killed a lot of people even those it wasn't as virulent as the flu strains of the last months of 1918.
Thanks VR. A couple of things from the Kolata book (I read it when it first came out, it's been a while); the 1918 flu was the worst of, and right in the middle of a five year period of particularly bad flu seasons (1916-1920); also, there was a very bad outbreak in the 1890s, one of the worst ever, but anyone who had that one and survived it never even got the sniffles during the Spanish Lady outbreak.