The book was terribly (adverb, "terrible" is an adjective and in this sentence "sad" is the adj so you need to use an adverb) sad but I liked it very much.
You can also say: Although the book was terribly sad, I enjoyed it very much.
1."Although" it rained a lot, we enjoyedour vacation.
2.I didn't get the job "although" I was extremely qualified.