The most authoritative source on DICOM is the free standard:
https://www.dicomstandard.org/current/
It is not particularly easy to read and thus not a great starting place.
The DICOM forum is a good place to ask questions:
https://groups.google.com/forum/#!forum/comp.protocols.dicom
David Clunie, a very nice guy and the editor of the DICOM standard, is active on this forum. He also has is own website that has many other links:
If you are interested in parsing or generating DICOM files, our DICOM standard browser is a great resource:
https://dicom.innolitics.com/ciods
I also wrote a very brief article about DICOM, which was meant to be the first in a series of articles, but alas I have been to busy to continue them:
https://innolitics.com/articles/dicom-i-facilitating-interop...
Oh, and here is a pretty good blog about DICOM:
http://dicomiseasy.blogspot.com
Checkout his "DICOM Tutorial" in the sidebar.
If you have any particular questions about DICOM, message me and I would be happy to try and answer them.