Reddit mentions of Concise Intro Theory of Numbers

I taught myself elementary Number Theory out of his book A Concise Introduction to the Theory of Numbers. It's a pretty dense little book, probably a little too intimidating for most beginning students unless you're willing to expound upon it, but it's nice to have a fairly dense number theory course that fits in a coat pocket.

Sorry to hear of his passing.

There's loads of good entry-level number theory books. I would seriously recommend this one

It looks short but covers quite a lot considering that including constructing a transcendental number and the group law on elliptic curves.