The Indian in the Cupboard Series by Lynne Reid Banks
The Indian in the Cupboard
The Return of the Indian
The Secret of the Indian
The Mystery of the Cupboard
The Key to the Indian
Like many series (including Prydain, above) my mother only read us aloud the first of these, and we went on to read others ourselves. I think my brother and I only made it up to the third in the series, because the rest were published when we were a little older and had lost interest.
A nice, creative series, though, and one that I think would be particularly good for young boys or reluctant readers. It's the story of Omri, who discovers that an old cupboard and key can bring plastic figures to life and through time when locked inside. Most notably, he brings to life Little Bear, a 19th century Indian.
The book has fun with differences both cultural, generational, and fantasical between the two as they eventually become friends. The difference is size leads to some Borrowers-esque creativity, but most of the conflict comes from the unwise re-enlivening of other plastic figures.
No explanation is ever given (as far I as know) for the magical properties of the cupboard, but if you can suspend your disbelief enough to swallow the premise, it's good fun.