The Secret Book of Flora Lea by Patti Callahan Henry

 Hazel and Flora Lea are just two of the many children sent into the English countryside to escape the London Blitz in WWII. As the older sister, Hazel takes her job of protector seriously but at the same time entertains Flora Lea with the imaginary world of Whisperwood where they can be heroes in the adventures Hazel creates. And then Flora Lea disappears. Believed to be drowned in the nearby Thames which they often referred to as a river of stars in their stories, Hazel is devastated. Twenty years later Hazel comes across a beautifully illustrated book entitled Whisperwood and the River of Stars. Believing this to be too much of a coincidence, Hazel embarks on a journey to see if her long lost sister could in fact be alive.  There are quirky characters and a bit of a surprise in the end but mostly a warm, hopeful read.