I read Eleanor Cameron's A Room Made of Windows in complete innocence as a standalone and, having been informed several times over a period of several years that there were other Julia Redfern books, I finally acquired and read them a month or two ago. Windows spoils various aspects of all of them, but it doesn't actually matter very much. The series grows up with the protagonist -- I was startled that the first book, Julia's Magic, is essentially a middle-grade novea, whereas Windows I guess would be called YA but is not really categorizible at all in current terms. Cameron is good. I did read the Mushroom Planet books in the proper order, at least.

P.