. I hate all of Sam's "Teach yourself X in 21 days" books, but Sams C Primer Plus by Stephen Prata is a pretty good one. I think some colleges even use it as a text book. I've got the blue fifth edition. I've found that the Teach Yourself books cover topics by using one specific example. The C Primer Plus does a good job at explaining each topic, then goes in depth to explain common practices, things to avoid, and unique uses.
The Arduino uses Processing, which is converted to pure Java by the IDE. Learning C would be good to know if you plan on using other microcontrollers, but I don't think the ins and outs are essential knowledge. You might be better off (and have more fun) just reading the examples and reference on processing.org.
I haven't looked at my C Primer Plus book in a while. I might be able to send it to you if you want to pay shipping.