Where can I read about DER certificate structure - fields and its registered values?
ASN.1 is defined by the International Telecommunication Union (the ITU). ASN.1 stands for Abstract Syntax Notation One, and is a schema definition language. DER, as well as its base form BER, are serialization formats of the data defined by the ASN.1-specified schema.
ASN.1 and DER/CER/BER are specified in ITU-T-X series standards, available at the official ITU website. Information about OIDs are available at http://oid-info.com/