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.

せんしゅうは　おもしろい本をよみました。 せんしゅう　おもしろい本をよみました。 The both are grammatically correct but the nuances are different: The second one is just what you did; no other information. The first one including は sounds you may have read a different kind of book(s) or a different interesting book(s) in a different week(s). For example, 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。 --------------- せんしゅうは　わたしは　おもしろい本をよみました。 わたしは　せんしゅうは　おもしろい本をよみました。 せんしゅうは　おもしろい本を　わたしはよみました。 The first one sounds most natural to me. The second one sounds the same with the first. Putting わたしは could indicate that someone else may have read a different kind of book. The third one sounds more like that. For example, せんしゅうは　おもしろい本をわたしはよみました。かれは　こわい本をよみました。 P.S. The space positions are optional in your sentences just to read the sentences easily with Hiragana. You do not need spaces when you write Japanese with commonly used Kanji. Hope this helps.