I don't disagree with the above answer.
I just wanted to say that on the Title/Credits HTML page (which contains copyright, date, etc) I included this line:
The HomemadeApple Font embedded in this ebook is copyrighted 2010 by Font Diner, Inc. under a Apache 2.0 license. (and I linked to the license).
So, I am not a lawyer, etc. The basics of copyright law, though, are fairly straightforward: If you don't own the image/text/song/whatever, you can't use it, unless there's an explicit license in place that lets you do what you want to do. There are some specific exceptions in place, like parody (which is about creating transformative work from pre-existing ...
This answer does not present any legal advice.
In fact your post contains two questions.
Speaking of the youtube license part you should definetly add information about the country you want to publish the eBook.
CC licenses come with different characteristics, regulating the rights and responsibilities.
The use of CC licensed images may be limited in terms ...
As the license requirements state "easily viewed by the user" you should include the full license in the EPUB zip files META-INF directory as e.g. open_font_license.txt, in addition to that add a stanza:
<dc:license>Open Font Software distributed as per open_font_license.txt</dc:license>
to the META-INF/metadata.opf file.
The latter should ...
In many countries you don't need to register a copyright, but it is automatically granted to you. Therefore it is probably unnecessary for you to do anything. Typically you can declare it on the title page of the book.
As I had not seen this question, I have asked another one very similar somewhere else.
I have tested the solution proposed by Anthon, but the addition of a <dc:license> element into the content.opf is unfortunaltely pointed as an error when checking the file with Sigil and FlightCrew:
Error XML child not recognized: the <licence> element is ...