If you just want to see coins of Charles I then, as Paul says, Brooker (Coins of Charles I (1625 - 1649). ) is still a good reference, though black and white illustrations only. However it doesn't cover the post-Royal period.
Besly's, Coins and Medals of the English Civil War is a smaller volume but gives more history.
Both can be picked up second hand. The first for as low as £10-£12, the second might be trickier .. £15-£20.
One of the members here (who is also a dealer) has an interest in the period and has written a few articles: http://www.mhcoins.co.uk/category/articles/
Of course, the British Numismatic Journal has numerous articles which can be viewed online here: http://www.britnumsoc.org/publications/Digital BNJ.shtml You can link to a pdf of the Index of contents from that page. Pages 30-31 of the Index list Charles I related articles.