There are three different types of attributive adjective endings: 1) adjectives after ein-words (ein, kein, mein, etc.) 2) adjectives after der-words (der, dieser, alle, etc.) 3) unpreceded adjectives (if no article is used, but also after words like viele, einige or NUMBERS). For categories 1) and 2), all adjective endings in the plural are indeed "-en". However, we are dealing with category 3) here, because the word "zwei" is a number. In this category, there are different adjective endings in the plural. In the accusative plural as in your example, the ending is "-e". http://coerll.utexas.edu/gg/gr/adj_04.html OT: Why is "englische" translated as "British"???
Actually, my post referred to the fact that the model translation originally read "I have two British books" (this has apparently been changed now). "English" and "British" do not mean the same thing and IMO using these terms interchangeably is wrong, even though the German DUDEN dictionary has an entry on "englisch = britisch, usage: colloquial".
"Englisch" is only capitalised if it
a.) is a synonym for "the English language". In this case, it's capitalised because it's a noun.
Ex. Es ist schwer, Englisch zu lernen. (It's difficult to learn English).
b.) refers to the subject "English" that you can take at school or study at university. Again, "Englisch" is capitalised here because it's a noun.
c.) is part of a proper name, e.g. there is a park in Munich that is called "Englischer Garten".
Otherwise, it's not capitalised.