The process of learning or re-learning depends on the reason for which you want to do it. Since your on Scicomp, I assume that your needs are related to a scientific project (BE, Ms, PhD ...) that requires you to understand/write/use computational codes (based on finite volume, finite element ...) to implement a numerical model. Therefore my advice, if this is the case is that you do not need ebooks to get going in the beginning because they usually contain a lot of information, which sometimes make the learning process boring. You can start with the following:
I personally think tutorial sites like CProgramming and C_Programming are great for their simplicity, the step-by-step approach with nice examples along the way. After that, you should copy the examples and do some "experiments" to grasp the basics (not the basics tought to a 13-year old :) ): changing variable types, defining functions, dealing with input and output ...
On the side, it's always good to have reference guides. They tend to be heavy in details, but they're useful occasionally for some reminders or some technical detail.
For all your easy, moderate and difficult programming tasks, StackOverFlow can be very useful for both solving issues and learning, specially when you begin experimenting things and reach the limit of what you know