I have received the book "Proofs from the books" 4th edition by Springer, and I found 3 problems on Number Theory relate to L.Euler. They are "Representation number as sum of two squares", "Quadratic Reciprocity" and "Three times $\dfrac{\pi^2}{6}$".
When I watched the public lecture of Clay Institute: Clay Public Lecture-A Tribute to Euler,
I saw the problem of partitioning number, say briefly $O_{n}=D_{n}$ in which $O_n$ is the number of ways to write $n$ as sum of odd number that small than $n$, $D_{n}$ is the number of ways writing $n$ as sum of different number that all small than $n. Professor Dunham presented a very nice proof, also related to Euler(the video lecture is available on that web, so you should download and watch it).
So, there are two question. Do you think this proof should be included in the book "Proofs from the book" ?
Are there any other beautiful proofs you think that are not mentioned in "The Book" ? Paul Erdos has gone to Paradise, but I think we should continue his work, completing "The Book".