A bit late for this year perhaps, but here is a nice solution to the How to tell them about Santa question. (This is not my idea and I am sorry I cannot credit it, but I don't remember where I read it.)
When you sense the time is right you take your child to a coffee shop perhaps - somewhere 'grown up' and just with you, if you can. You say you have noticed how much they have grown up this past year, and you have often noticed how kind they can be, so you think perhaps they are old enough now to learn how to become a Santa.
You describe Santa's job - surprising people with thoughtful gifts and making them happy - and you suggest they give it a trial this Christmas by choosing someone (perhaps a neighbour, or a younger sibling?) figuring out what they might want or need, and getting it to them for Christmas without them knowing who sent it.
The example given was a solitary woman who spoke to no-one and was seen lifting in her morning milk from her doorstep in bare feet. The child had to guess what size slippers etc etc and deliver them without being seen on Christmas Eve. The writer descibed the look of delight on his child's face when seeing the woman come to her door wearing the slippers as 'unforgettable' and I can believe it. Whether apocryphal or not, the story appeals as an approach to 'telling the truth' about Father Christmas that may have a lot of mileage in it. It could be seen as a kind of right of passage, which would surely encourage thoughtfulness and unselfishness...
It would certainly be nicer than my experience when my naughty cousin told me 'the truth' in bed on Christmas Eve!
There are quite a few posts about children's Christmas books under Simply Christmas. Here is one with further links within if by chance you have any time to browse!