This trophy is highly missable if you own the dlc's as you can void this trophy giving the Astrologer the Notes about Obsidian you can dig up instead of the silver star book.
If you give him the notes, you'll get +10 affinity with the astrologer and you'll save one week of waiting for him, as if you give him a silver-star book, next week he will ask for a gold-star book.
This gold-star book is the one that counts for the trophy, as selling him gold-star books doesn't count.