THE definitive book on the subject of the 1918 influenza pandemic. More recent and way more detailed (and referenced!) than Gina Kolata's book, Flu, which I also read.
The particular value of Barry's book, in addition to tracing the actual courses of the global waves of infection (there were at least 3, with #2 being "the big one"), is that he puts the events in context: World War I was in high gear, requiring that lots of young bodies be in crowded conditions and that "morale" be upheld at all costs; Tammany Hall was in full swing, making public health appointments disastrously political; and no clinical research or drug regulatory infrastructure existed yet, leading people to sometimes work at cross-purposes at best.
The text does get bogged down in places, but you can skim without losing much of the narrative. It's just that the disease and its effects were so overwhelming: influenza killed up to 5% of the WORLD's population within a year. Even now, influenza and pneumonia kill >50,000 people in the U.S. alone each year--it's the 8th leading cause of death--and the incidence is not going down. All it takes is a little shift in the antigens, a jump from an animal to a human, and off we go again...
I was stupid enough to no get my flu shot last year. Never again.