This is a book that may be British. It may have Rebecca or Dreams in title. Or Emma or Milly. It’s a chapter book with some illustrations. A girl visits a dream world where there are friends or helpers. One is a tall man (maybe a giant) with a large furry coat. It may be yellow. There may be a room she’s in that’s like being inside a globe, with lots of locks. Also, she may have to solve a series of puzzles to go through this world.
Perhaps “Rebecca’s World” by Terry Nation?