1) All articles can be used as demonstrative pronouns, in strength between personal pronouns and "dieser/diese/dieses". This is easy to see, beause in this case they don't belong to a noun. So alternatively, you could write "Dieses sind interessante Bücher" (but that would normally not be used, because it's too strong). 2) Yes, "viele Freunde" is accusative plural. The words "nichts, wenig, etwas, viel, genug" are not real adjectives, and they are not inflected at all, though "wenig" and "viel" can be put in the plural. Any adjective that would follow "viel(e)" would use strong inflection: "Sie hat viele gute Freunde". BTW, "das soziale Mädchen" is not something you'd say in German. 3) Correct is "Ich bin kein Experte". "Expert" is a mistake.