we've encountered her before!
This is a very short book, just 63 pages (including chapters and full-page illustrations) so I won't be able to say too much without giving the plot away entirely.
Ronnie lives with his aunt (circumstances undisclosed), who used to be married to a lighthouse keeper and had lived with him at an isolated lighthouse, the titular Tern Rock, for decades. The current keeper approaches her one day and asks if she would be able to take Robbie and man (woman?) the light for him for two weeks so he can visit family in Nova Scotia in early December. He insists that he will return to relieve them well in advance of Christmas. Aunt Martha loved her time at the lighthouse and, with Ronnie's agreement, makes plans to relieve Mr. Flagg.
Ronnie quickly learns the routines of the lighthouse, and finds his days full between caring for it with his aunt, keeping the house tended, and keeping up with his school lessons. But when December 15th arrives, the promised homecoming date, Mr. Flagg is nowhere to be seen. Ronnie becomes more and more bitter as he realizes that he may be stuck at the lighthouse for Christmas. How can he and his aunt possibly celebrate properly so far from home?
This slim, quick read would be great for beginning readers.