Start with some school textbooks for grades 8-12 i.e. Secondary Education. This is more for a refresher course in the absolute basics.
The above can be supplemented with the following books to develop intuition;
1) Who is Fourier - https://www.amazon.com/Who-Fourier-Mathematical-Adventure-2n...
2) Functions and Graphs - https://www.amazon.com/Functions-Graphs-Dover-Books-Mathemat...
After this is when you enter undergraduate studies and you have to fight the dragon of "Modern Maths" which is more abstract and conceptual. In addition to standard textbooks; i suggest the following;
1) Concepts of Modern Mathematics - https://www.amazon.com/Concepts-Modern-Mathematics-Dover-Boo...
2) Mathematics: Its Content, Methods and Meaning - https://www.amazon.com/Concepts-Modern-Mathematics-Dover-Boo...
3) Mathematical Techniques (i am linking this so you can see the reviews but get the latest edition) - https://www.amazon.com/Mathematical-Techniques-Dominic-Jorda...
Finally, if you would like to learn about all the new-fangled mathematics your best bets are;
a) The Princeton Companion to Mathematics - https://www.amazon.com/Princeton-Companion-Mathematics-Timot...
b) The Princeton Companion to Applied Mathematics - https://www.amazon.com/Princeton-Companion-Applied-Mathemati...
One important piece of advice that i have is to become comfortable with the Symbols, Notation and Formalism used in Mathematics. Most students are intimidated by the Formalism (which is nothing more than a precise form of shorthand to express abstract concepts) and give up on studying Mathematics altogether. This is a shame since it is merely the Form and not the Function of Mathematics.
Concepts of Modern Mathematics by Stewart
https://www.amazon.com/Concepts-Modern-Mathematics-Dover-Boo...
Dover Version with Google Preview Button
http://store.doverpublications.com/0486284247.html
Introduction to Mathematical Reasoning: Numbers, Sets, and Functions
https://www.amazon.com/Introduction-Mathematical-Reasoning-N...
How to Prove It by Velleman
https://www.amazon.com/How-Prove-Structured-Approach-2nd/dp/...
[1]http://www.amazon.com/Mathematical-Ideas-Edition-Charles-Mil...
[2]http://www.amazon.com/Concepts-Modern-Mathematics-Dover-Book...
[3]http://www.amazon.com/Schaums-Outline-Discrete-Mathematics-R...
From the Amazon description:
In this charming volume, a noted English mathematician uses humor and anecdote to illuminate the concepts underlying "new math": groups, sets, subsets, topology, Boolean algebra, and other subjects. No advanced mathematical background is needed to follow thought-provoking discussions of such topics as functions, symmetry, axiomatics, counting, topology, hyperspace, linear algebra, and more. 200 illustrations.
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[1]: http://www.amazon.com/Concepts-Modern-Mathematics-Dover-Book...
here's a whirlwind tour http://www.amazon.com/Concepts-Modern-Mathematics-Ian-Stewar...
http://www.amazon.com/Concepts-Modern-Mathematics-Ian-Stewar...
For access to a lot of mathematical concepts at a reasonable reading level, not at all expensive, I recommend Concepts of Modern Mathematics by Ian Stewart.
http://www.amazon.com/Concepts-Modern-Mathematics-Ian-Stewar...
Ian Stewart is a mathematician who loves to write popular writings on mathematics, and you can hardly go wrong with anything he has written.
From Zero to Infinity: What Makes Numbers Interesting by Constance Reid
http://www.amazon.com/Zero-Infinity-Makes-Numbers-Interestin...
is very accessible and covers a number of interesting topics.
The Art of Problem Solving by Richard Rusczyk and Sandor Lehoczky
http://www.artofproblemsolving.com/Store/viewitem.php?item=p...
http://www.artofproblemsolving.com/Store/viewitem.php?item=p...
is a straight-up contest preparation book, in two volumes, that your son may find interesting. Volume 2 is for high school level contests.
For an interesting (in places laugh-out-loud funny) book about the place of mathematics in modern life and how mathematicians think about mathematics, I recommend The Pleasures of Counting by T. W. Körner.
http://www.amazon.com/Pleasures-Counting-T-246-rner/dp/05215...
This one is more challenging as to reading level and as to mathematical level than the recommendations above, but well worth having around the house.
I'll recommend a couple of books from that thread:
http://www.springer.com/physics/book/978-0-306-45036-5
http://www.amazon.com/Mathematics-Short-Introduction-Timothy...
I agree with the recommendation of An Introduction to Mathematical Reasoning in this thread.
Another participant has already recommended my favorite for background reading, Concepts of Modern Mathematics by Ian Stewart.
http://www.amazon.com/Concepts-Modern-Mathematics-Ian-Stewar...
Get that right away.
Sawyer's A Mathematician's Delight is surely also good (I've read other books by Sawyer).
http://www.amazon.co.uk/gp/product/0486462404/
Read those for background as you get my favorite overviews of mathematics: Basic Mathematics by Serge Lang and Numbers and Geometry by Joseph Stillwell.
http://www.amazon.com/Basic-Mathematics-Serge-Lang/dp/038796...
(Basic Mathematics is mostly high school level math, with a minimum of fuss and bother, and good exercises.)
http://www.amazon.com/Numbers-Geometry-John-Stillwell/dp/038...
(Numbers and Geometry is mostly undergraduate level math, with very good explanations and excellent exercises.)
Concepts of Modern Mathematics by Ian Stewart
http://www.amazon.com/Concepts-Modern-Mathematics-Ian-Stewar...
Numbers and Geometry by John Stillwell.
http://www.amazon.com/Numbers-Geometry-John-Stillwell/dp/038...
The Pleasures of Counting by T. W. Körner
http://www.amazon.com/Pleasures-Counting-T-W-K%C3%B6rner/dp/...
Mathematics: A Very Short Introduction by Timothy Gowers
http://www.amazon.com/Mathematics-Short-Introduction-Timothy...
- Concepts of Modern Mathematics - https://www.amazon.com/Concepts-Modern-Mathematics-Dover-Boo...
- Methods of Mathematics Applied to Calculus, Probability, and Statistics - https://www.amazon.com/Methods-Mathematics-Calculus-Probabil... (all books by Richard Hamming are recommended)
- Calculus: An Intuitive and Physical Approach - https://www.amazon.com/Calculus-Intuitive-Physical-Approach-...
For a Textbook reference, the following are quite good;
- Mathematical Techniques: An Introduction for the Engineering, Physical, and Mathematical Sciences - https://www.amazon.com/Mathematical-Techniques-Introduction-... (easy to read and succinct)
- Mathematics for Physicists: Introductory Concepts and Methods - https://www.amazon.com/Mathematics-Physicists-Introductory-C...
For General reading (all these authors other books are also worth checking out);
- Mathematics, Queen and Servant of Science - https://www.amazon.com/Mathematics-Queen-Servant-Science-Tem...
- Mathematics and the Physical World - https://www.amazon.com/Mathematics-Physical-World-Dover-Book...
- Mathematician's Delight - https://www.amazon.com/Mathematicians-Delight-Dover-Books-Ma...