A message for Santa by Hiawyn Oram & Tony Ross (Andersen Press, 1995).
Emily loves Christmas, but the thought of Santa sneaking into her house at night terrifies her. So, she leaves him a message... yes, he can come in - by the back door - but no further than the kitchen table. Santa obliges, and Emily begins to change her mind.
See - Christmas and Santa can be scary - it's not always sweetness & light, so it's great to see books which acknowledge, and ease, children's anxieties.