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But note: though we're fairly strong on security issues, we are not crypto experts.
If you like your guide to cryptographic hashes to be less illustrated, and written by crypto experts:
http://cacr.uwaterloo.ca/hac/about/chap9.pdf
More reading applicable to Bitcoin would be chapter 11 on digital signatures (here, just get the whole book:).
Handbook Of Applied Cryptography (Mit Press).pdf
Note, ECDSA is merely a footnote in this text, you can build on your knowledge in this order:
Intro to Elliptic Curve Crypto:
http://www.fujitsu.com/downloads/MAG/vol36-2/paper05.pdf
A longer history:
http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.215.1721&rep=rep1&type=pdf
and the initial paper on ECC:
http://www.ams.org/journals/mcom/1987-48-177/S0025-5718-1987-0866109-5/S0025-5718-1987-0866109-5.pdf
Then try chapter 6 of Course in Number Theory and Cryptography - Koblitz (zee pirate link)
If you want to understand the above, have some graduate mathematics prerequisites:
Algebraic Geometry:
http://www.math.lsa.umich.edu/~wfulton/CurveBook.pdf
Elliptic curves and modular forms:
http://w3.impa.br/~hossein/courses/material/arithmetic.pdf
