Spies, suspense, intrigue - and a tough, likable heroine; Night Sky, by Clare Francis. New York: William Morrow & Co. 631 pp. $16.95.

This is a conventional World War II novel with several unconventional features. Set in England, France, and Germany between 1935 and 1945, ''Night Sky'' includes all the usual ingredients - spies, intrigue, suspense, warfare, romance, an attractive heroine, a likable hero, a villain you love to hate, and a happy ending.

But what is unusual about ''Night Sky'' is author Clare Francis herself. Women don't usually write novels about World War II naval warfare.

There are exceptions, of course, such as Helen MacInnes, but such writers are rare indeed.

Unusual as well is the central character - the likable Julie Lescaux, a woman. In most novels like ''Night Sky'' women do play a role, but not the main one.

Julie Lescaux is a young Englishwoman who goes to live with relatives in Brittany in 1935 to bear her illegitimate child. Accepted there, she stays on with her son, Peter.

Suddenly, there is war, and it is too late for them to return to England. Julie becomes involved in an escape line that, with the assistance of the British Navy and MI9, returns downed Allied airmen across the channel in large fishing boats.

I enjoyed reading about this heroine doing some brave, daring, dangerous things some real-life women did during World War II. And she is just one of the surprises that contributed to my enjoyment of ''Night Sky.''

I could be critical of some of the more conventional aspects of the novel, of some not-totally-believable characters, or of some incredible plot coincidences, but I'm not going to be. I enjoyed ''Night Sky'' too much to worry about such details.