The way into set theory
Here are four suggestions (two "entry level" books, and two just a notch up in difficulty):
- Herbert B. Enderton, The Elements of Set Theory (Academic Press, 1997) is particularly clear, and nicely marks off the informal development of the theory of sets, cardinals, ordinals etc. (guided by the conception of sets as constructed in a cumulative hierarchy) and the formal axiomatization of ZFC. It is also particularly good and non-confusing about what is involved in (apparent) talk of classes which are too big to be sets – something that can mystify beginners. It is written with a certain lightness of touch and proofs are often presented in particularly well-signposted stages. The last couple of chapters or so perhaps do get a bit tougher, but overall this really is quite exemplary exposition.
- Derek Goldrei, Classic Set Theory (Chapman & Hall/CRC 1996) is written by a staff tutor at the Open University in the UK and has the subtitle ‘For guided independent study’. It is as you might expect extremely clear, it is quite attractively written (as set theory books go!), and is indeed very well-structured for independent reading. The coverage is very similar to Enderton’s, and either book makes a fine introduction.
- Karel Hrbacek and Thomas Jech, Introduction to Set Theory (Marcel Dekker, 3rd edition 1999). This goes a bit further than Enderton or Goldrei (more so in the 3rd edition than earlier ones). The final chapter gives a remarkably accessible glimpse ahead towards large cardinal axioms and independence proofs. Again this is a very nicely put together book, and recommended if you want to consolidate your understanding after one of the first two books by reading another presentation of the basics and want then to push on just a bit. (Jech is of course a major author on set theory, and Hrbacek once won a AMA prize for maths writing.)
- Yiannis Moschovakis, Notes on Set Theory (Springer, 2nd edition 2006). A slightly more individual path through the material than the previously books mentioned, again with glimpses ahead and again attractively written.
All these books are in print, though none are cheap: indeed, Enderton’s is quite absurdly expensive. But all are ‘musts’ for any university library and are widely available. I’d strongly advise reading one of the first pair and then one of the second pair. (And do this before tackling more advanced books like Kunen's or the Jech bible which go more a lot more quickly through the basics and then deal with more advanced topics including forcing and large cardinals.)
I'd recommend "Naive Set Theory" by Halmos. It is a fun read, in a leisurely style, starts from the axioms and prove the Axiom of Choice.
Also, see this XKCD. http://xkcd.com/982/