Hi,
I'm KGS 7kyu and i have just read two times the book Graded Go Problems for Beginners vol.2.
Now i'm doing the volume 3, the following book of the series.
I would like to ask you if you can suggest some tsumego books with problems of the same level of the GGPFB vol.3 since i would like to do some other problems of the same rank before doing again the ones in the book i own.

Is this book good for 7kyu rank or is it more difficult?
Are the problems in the book divided into levels as on Graded go Problems serie?

I was around 7 kyu before I took a long break, I've returned now and this is the exact book I'm using for tsumego. It's perfect for that level, I think. It's not divided into levels, but it has 3 big chapters: 1 move problems, 3 move problems and then 5 move problems.

Do you know if there are other books similar to Graded Go Problems with sections divided into subjects (i.e. middle game problems, life and death, etc)?

Graded Go Problems are generic with various topics; I cannot recommend other such books for KGS 7 kyu level. However, there are good other (problem) books with specific topics suitable also for that level: First Life and Death, Life and Death Problems 1 - Basics, Tactical Reading, Tesuji (Davies).

I also highly recommend this book. However, it is not the same kind of book as the Graded Go Problems. To me, Graded Go Problems seems like a collection of problems that you might easily encounter in your game, so knowing those shapes is very helpful. On the other hand, 1001 L&D Problems contains completely artificial problems. You'll likely never encounter them in your games. For me, they were useful in improving my reading, precisely because I could not rely on having seen the shape before.