'Attack Alarm' was one of Innes' first books, written when he was a young gunner in WW2. Drawing on his wartime experience, Innes uses the Battle of Britain as the backdrop to a planned German assault on English airbases aided by fifth columnists. When his gun crew shoots down a German bomber, Barry Hanson overhears the downed pilot boast of a planned aerial attack on English airbases, only to stop dead when he spots the base's librarian, Vaile. With his suspicions about Vaile ignored by his superiors, Hanson must rely on his own wits and the assistance of WAAF Marion Sheldon to find out the truth.
In the dedication page, Innes apologises for the rushed nature of the book, noting he composed it during his time serving in WW2. And it is true that at this early stage, his storytelling ability is not fully developed yet, and the more epic scope of his later works is absent. However, his knowledge of the workings of an airbase in wartime is used well, describing several German air attacks in exciting fashion. And even at this early stage some Innes trademarks are present. Barry Hanson is the typical Hammond Innes protagonist; an ordinary "everyman" who unwittingly gets dragged deeper and deeper into a larger conspiracy. WAAF Sheldon is no mere love interest, she's a capable and resourceful heroine who risks punishment to help Hanson. And Vaile is an intriguing character who may or may not be an antagonist. Given the only evidence against him is a German pilot shooting his mouth off, we're never sure whether Vaile is a Nazi spy or innocent fall-guy. The mid-book confrontation between Hanson and Vaile muddies the waters further, and it's only at the end that we learn the truth.
'Attack Alarm' doesn't quite have the depth and skill of his later books, but the combination of warime action and fifth-column intrigue works well. If you can allow for some rough edges, it is as good a place as any to introduce yourself to Hammond Innes.