In addition to those mentioned, here is a good book which is just under $30:

http://www.amazon.com/How-Think-Like-Mathematician-Undergraduate/dp/052171978X/ref=pd_sim_b_5

In addition to those mentioned, here is a good book which is just under $30:

Kevin Houston, How to Think Like a Mathematician: A Companion to Undergraduate Mathematics, 1st Edition 2009.

In addition to those mentioned, here are some good books in a reasonable price range:

http://www.amazon.com/How-Think-Like-Mathematician-Undergraduate/dp/052171978X/ref=pd_sim_b_5

http://www.amazon.com/Introduction-Mathematical-Reasoning-Peter-Eccles/dp/0521597188/ref=pd_sim_b_33

In addition to those mentioned, here is a good book which is just under $30:

http://www.amazon.com/How-Think-Like-Mathematician-Undergraduate/dp/052171978X/ref=pd_sim_b_5