Tawheed, attributes of Allah, the Prophets, and the Last Day.
The different types of water and their validity for purification. arrisaalah almufeedah pdf link
In many traditional Islamic seminaries, this book serves as a stepping stone. Once a student masters a smaller text (like Safinah An-Najah ), they move to Ar-Risaalah Al-Mufeedah before tackling the more complex commentaries. Tawheed, attributes of Allah, the Prophets, and the Last Day