I think both are grammatically correct, but the second one sounds more natural to me. ----------------------------------------------------- The first one would sound more natural if it's followed by more information such as "I read a scary book this week." "せんしゅう　は　おもしろい　本　を　よみました。こんしゅう は　こわい　本　を　よみました。” This example given in the post above would sound perfectly natural. ------------------------------------------------------ If you'd like to add "watashi" in the sentence, you could say　 ”せんしゅう　わたし　は　おもしろい　本　を　よみました。” ”わたし　は　せんしゅう　おもしろい　本　を　よみました。” They both mean "I read an interesting book last week." In the first sentence, "last week" is slightly more emphasized and in the second one, it's more focused on the fact that YOU read it. I put spaces to make it easier to read, but you can opt it out. Did it make sense? I hope this helps.