The books concerning the science of Tawheed and Islaamic fiqh are many, old and new. And those that I direct you to read are as follows:
Al-Usooluth-Thalaathah, Kashfush-Shubuhaat, al-Qawaa’idul-Arba’, Usoolul-Eemaanis-Sittah, Masaa‘ilul-Jaahiliyyah and Kitaabut-Tawheed, and all of these are by Shaykhul-Islaam Muhammad Ibn ’Abdul-Wahhaab (d.1206H) – rahimahullaah. After these, there is al-’Aqeedatul-Waasitiyyah and Kitaabul-Hamawiyyah, and both of them are by Shaykhul-Islaam Ibn Taymiyyah – rahimahullaah. And there is an-Nooniyyah by Ibnul-Qayyim (d.751H), along with its explanation. There is al-Hayaat fee Dhillil-’Aqeedatil-Islaamiyyah by the writer of these words. There is al-Irshaad by Shaykh Saalih al-Fawzaan, and al-’Aqeedatus-Saheehah wa maa Yudaaduhaa by the Noble Shaykh, ’Abdul-’Azeez Ibn Baaz (d.1420H) – yarhamuhullaah. And there is also al-Qawlul-Mufeed by al-Fawzaan and Kitaabut-Tawheed by Ibn Khuzaymah (d.311H). And the books of the recorded Sunnah concerning the science of creed (al-i’tiqaad) are many. Indeed, I have recently mentioned some of them in the past.
As for the books of Islaamic fiqh, then likewise, the books are many. However, you must begin with al-’Uddah Sharh Kitaabil-’Umdah and the shorter books of the Hanaabilah. And there is al-Mulakhkhasul-Fiqhee by al-Fawzaan. And this must be followed by the books of hadeeth that have been explained, such as Sharh ’Umdatul-Ahkaam and Subulus-Salaam ’alaa Bulooghil-Maraam and al-Muntaqaa and its explanations, such as Naylul-Awtaar. Then, you must move onto reading the books along with striving hard in consulting the guidance of the Salaf concerning the book that you love to read, even if it be by way of the telephone or sending letters. May Allaah grant everyone success in whatever He is pleased with.