predicative adjektives: not declinable:
Das Buch ist deutsch | The book is german
attributive adjectives: declinable
Das deutsche Buch | The german book
predicative adjectives are linked to their noun by a copula (Verbs like sein, werden, bleiben | to be, to become, to stay).
attributiv adjectives don't need a copula and are linked directly to the noun.
So in this case, "deutsch" must be declined.
if it has no definite article (das in this case) or the indefinite article doesn't end in the same sound as the definite, (den/einen, dem/einem etc.), you add the appropriate gender ending. Er for masculine, e for feminine, and es for neuter. (may be wrong) That's mainly for nominative, and it gets more complex with the other cases.
Berlitz Self-Teachers are good, so are "Teach Yourself" books, especially the ones from the 60s through the early 90s (they go back to the 40s and are still making decent books). Both are easy to find cheap online. Other than that I would recommend Hugo or Assimil, harder to find for dirt cheap but also very good. The German company Buske makes language books, mostly in German for other languages but a few are in English. I've found their books to be particularly expensive but they have very good books for certain languages which are hard to come by. Overall, your best option is Berlitz Self-Teachers and the TYS series, and I would suggest searching on thriftbooks.com before spending more elsewhere. Exercise books may already be filled in, but the self-teachers aren't exercise books and out of over a dozen books ordered from that website, including 7 language books, only 1 had any writing in it and it didn't degrade it in the slightest. But I would suggest sticking to books rated "good" or "very good" condition instead of acceptable.
Like the English "a German book", "ein deutsches Buch" can mean a book from Germany as well as a book in German. That's why this is the best translation. "a book in German" is too narrow, because it covers only one of the two possibilities.
If you want to express "a book in German" or "a book from Germany", you can say "ein Buch auf/in Deutsch" resp. "ein Buch aus Deutschland".