The Boggart by Susan Cooper

Twelve year old Emily and her brother Jess manage to be regular kids, even after their family has inherited a real, old, damp, drafty Scottish castle. They visit it during the summer to tend to details, like caregivers and fix-up work, and they bring some souvenirs home to Canada. Including, unknown to them, a mysterious…something. Ice cream and peanut butter start to disappear, and things that never went bump in the night are doing it now. What was in that desk that Emily brought home from Scotland? Whatever it is, it’s discovering electricity for the first time, as it tries to communicate and play jokes on Emily and Jess through their computer, the lights in a theater, and worst of all, traffic lights… This supernatural tale combines science and legend with humor and very believable consequences.

Author's web site:

http://www.thelostland.com/bogg.htm

The Lost Land is Susan Cooper’s web page. It has lots of information about her other books, including the Dark is Rising Series.