The Indisputable Existence of Santa Claus by Hannah Fry and Thomas Oléron Evans is a fun light-hearted look at the mathematics of all things Christmas. I mean, it's a serious academic work. One of the two.
If the current month is ≥10 then you should definitely buy this book. For all other months it depends on how much you really love Christmas. Or maths. Or some sufficient combination of the two.
This is the new 2017 edition, containing "more merry maths" (it's not clear if this is a larger quantity of merry maths, or maths with a higher merriness quotient), with a redesigned shiny red cover, and the book is signed by both authors.
Price = p168 pence = £9.97