If you're interested of learning more about ASN.1, two great books on the subject are:

- ASN.1 Communication between Heterogeneous Systems[0]

- ASN.1 Complete[1]

Both of which are available online for free. Last time I checked, they may not cover the latest fads like JER etc. but will provide you with a solid understanding of ASN.1 nonetheless.

[0] https://www.oss.com/asn1/resources/books-whitepapers-pubs/la... [1] https://www.oss.com/asn1/resources/books-whitepapers-pubs/du...