Could you be more specific regarding "CS fundamentals"? Getting better at programming and getting better at algorithms can be entirely different endeavors. On the one hand, there is Guttag's Intro to Comp Sci book (the edX MOOC as a whole is, in my opinion, the best MOOC on introductory CS using Python). Books like CLRS (mentioned by kdavis) will teach you how to construct efficient algorithms but won't teach you how to program per se (all in pseudocode).
