From the back cover:

American fliers arriving in Europe from September 1917 brought with them no aircraft. Instead, US units had to obtain machines mainly from the British and French. From early 1918 American pilots were issued with SPAD fighters and they never looked back. As this volume details, the first American trained pilot to become an ace was Lt Douglas Campbell, who shot down five German aircraft by the end of May 1918. He was a member of the celebrated 94th 'Hat in the Ring' Aero Squadron, which created the bulk of American aces in World War I.