The two sentences have a slight difference in emphasis due to the placement of the pronoun "ben" (I).
- "Ben bu kitabı okuyorum." - In this sentence, the emphasis is on the person (I) who is reading the book. It emphasizes that it is specifically "I" who is reading the book.
- "Bu kitabı ben okuyorum." - In this sentence, the emphasis is on the book itself. It emphasizes that it is specifically "this book" that "I" am reading.
Both sentences essentially mean "I am reading this book," but the emphasis shifts between the person and the book depending on the word order.