Is there any good "basic" books on number theory?
If by "basic" you mean elementary, try Courant's "What is mathematics".
Its a classic text and its cheap to buy. Its got very good stuff on our number system, from natural numbers, primes, rationals, and irrationals. Its one of the best books I have ever read. Its not a book exclusivley on number theory though, rather it has chapters on a bunch of interesting math bits. More of a "intro to abstraction" type of book. Check it out, its a damn good read.
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