What are the differences and/or similarities between scholarly
definitions/discussions of ethnic and national identities? Is there
any bibliographic material which addresses this topic directly that I
could consult? I am interested in particular by how identity,
(national or ethnic ?) is constructed and negotiated linguistically.

The best resource for understanding identity is Karen Tracy's book,
"Everyday Talk: Building and Reflecting Identities." I'm not familiar
with this book, "The Discursive Construction of National Identity,"
Edited by Ruth Wodak, Rudolf de Cillia, Martin Reisigl, and Karin
Liebhart but it is using the right terms: "discursive construction of
(social) identity," a field focused on how we build "who we are" in
our talk and recognize it in others.  The method typically used for
studying identity from this viewpoint is called discourse analysis
(actually several methods under this umbrella).

Look up the word ethnic in the dictionary - we are all ethnic, it
means any group with common heritage / identity. The  word ethnic has
been / is miss-used in our contemporay pop-culture.

In reality there is no seperate race of humans - er're all from Africa
and simply travelled around the globe in groups and grew in different
areas. There can be more DNA in common with the darkest African in
Botswana  and the lightest Swede than there is between to people in
the same town sitting at the same bus stop many times.

Check the answer in "Google Answers" to a question regarding African
Heritage.