Welcome to the Arduino world,
Please recommend a kit for a newbee with a Mac. One that he won't outgrow soon and that can be added to
Imho any kit will do, and the more time you spend with your arduino the faster you will outgrow the kit as you have made all possible "apps". Any kit can be added to depending on your interests, look at the - http://www.pololu.com/ - or - http://www.sparkfun.com - for more toys than one can dream of, again it depends on your interests.
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http://www.earthshineelectronics.com/files/ASKManualRev4.pdf - is a nice free book but there are more.
It is also good to spend some time on the tutotial section and the reference section as there is a lot to learn. Also the playground is interesting but I would advise to check the interesting parts after reading the tutorial & ref section.
Leaves open one question : what do you want to build ?