"The Shortest Day: Celebrating The Winter Solstice" by Wendy Pfeffer is a wonderful book that explains the winter solstice, and also the traditions throughout history and different cultures.
There are activities at the back of the book as well, such as measuring shadows on the shortest day, hosting a solstice party for your friends and also for the birds. A short list of further reading suggestions and web sites can be found on the last page.
"Over 2000 years ago, Romans celebrated the shortest day with festivals and merrymaking. They gave evergreen branches to friends as a sign of good luck. Evergreen wreaths decorated their doors. Since these plants stayed green when others turned brown, they reminded the Romans of the coming spring."
Wendy Pfeffer, the author, has many other interesting titles, most related to science and nature. She also has written books about the Spring Equinox, the Summer Solstice, and the Fall Equinox.