i think handsome in meaning looking well can be said only about men, if handsome is a woman or a thing, it has a different meaning, as this thing or woman is imposing, not that it looks really well and יפה means something looks well, so beautiful or pretty is the best translation, if it would a sentence about some man, it could be handsome, but not a book.
The Hebrew course is relatively new. One of the course creators has put the letters and vocab on Memrise, which you might find helpful. See:
https://www.memrise.com/course/1087087/hebrew-alef-bet-print/ for the letters, and
https://www.memrise.com/course/1031737/hebrew-duolingo/ for the rest of the words.
Also, you can try to give feedback to the developers by flagging an exercise or for more general comments like yours, maybe use the "contact us" page https://support.duolingo.com/hc/en-us
I actually have a spreadsheet on my computer for the Hebrew course because I'll never remember how to spell anything otherwise. Every time I learn a new word, I put it in my list along with info regarding its gender, number, and sometimes its part of speech.
It is pronounced towards the back of mouth, rather than the front. I've read several other learners likening it to a French 'r'. Not having learnt French, I can't say if it is. German 'r's are as variable as English ones (English is a Germanic language) so I wouldn't liken the Hebrew 'r' to the German except for words that have come into Hebrew from German (often via Yiddish).
For a precise answer you need a linguist, which I'm not, but I'm a native Hebrew speaker, studied German and French, and traveled some in France and Germany. AFAICT the (common modern) Hebrew and (common? standard?) German /r/ are identical. The french tends, at least in some words ("Sartre" is my favorite), to be different; I'm not sure but maybe it's unvoiced? It sounds to me more like the Hebrew ח (or German (a)ch).
For appearance, we would usually say "It is a pretty book." If it were very pretty we could also say "It is a beautiful book". We could use "handsome", but would rarely do so. For content, we would usually say "It is a good/excellent book." If the content touched us emotionally, we might also say "It is a beautiful book" or "It is a lovely book". "Pretty" would never be used for content, though.