Marco's Millions Series by William Sleator

When Marco's shy and fragile little sister, Lilly, tells him about the strange lights in the basement, he thinks she's imagining things. But Marco is curious, and before long he investigates. He finds a passage into a strange world, where he meets insect-like creatures who seem to know all about Lilly. They believe she is special and beg Marco to bring her to them so that she can embark on a dangerous quest to save their world from ruin... moreWhen Marco's shy and fragile little sister, Lilly, tells him about the strange lights in the basement, he thinks she's imagining things. But Marco is curious, and before long he investigates. He finds a passage into a strange world, where he meets insect-like creatures who seem to know all about Lilly. They believe she is special and beg Marco to bring her to them so that she can embark on a dangerous quest to save their world from ruin. Can Marco help the creatures while protecting Lilly from harm? less

Orphan Anne Levi tolerates her distant Aunt Ruth, with whom she lives, but adores her mysterious Uncle Marco, who flits in and out of their lives at irregular intervals. When he gives Anne two unusual boxes with strict instructions not to open them, curiosity gets the better of her... moreOrphan Anne Levi tolerates her distant Aunt Ruth, with whom she lives, but adores her mysterious Uncle Marco, who flits in and out of their lives at irregular intervals. When he gives Anne two unusual boxes with strict instructions not to open them, curiosity gets the better of her. Opening the first one, she releases an unusual crablike creature that grows and reproduces rapidly; the life form and its offspring construct a fantastic palace in the basement and communicate with Anne telepathically. Dismayed by what she has done, Anne opens the second box, which she had hidden in her closet, revealing a clocklike object that has the ability to slow down time at the basement creatures' request, but only when Anne agrees to carry messages between the creatures and the clock. Unfortunately, the owners of a suspicious development company are intrigued by the time slowdowns and increase their ominous efforts to control Anne, her home, and the strange devices within it.- Susan L. Rogers, Chestnut Hill Academy, PA (School Library Journal) less