Investigators of the Unknown Series by Janet Taylor Lisle

When nine-year-old Angela and her friends begin investigating the letters she has received from her fairy godmother, it helps take Angela's mind off her strained relationship with her father.

When Angela moves away for a year, her best friends Poco and Georgina and a mysterious classmate are convinced that the elderly Miss Bone, caretaker of Angela's house, is responsible for the inexplicable disappearance of her cat.

Georgina and Poco try to help their friend Walter who is suffering from an identity crisis and receiving strange messages from his dead mother.

Because Angela seems to have become a totally different person and tells of being abducted by aliens and living among them, her friends change their minds about her.