What are the books that help a student of knowledge understand the books of the Salaf, especially if he finds difficulty understanding their language?

Seeking knowledge is best done orally, directly from scholars, and this is the highest form, as the student is influenced by the intonations and emotional expressions of the teacher. Next in order are audio recordings, then books. One who reads knowledge from books may gain much good, but it requires more time and understanding, lest the beginner misinterpret the intended meaning of a phrase. If understanding the books of the early righteous predecessors proves difficult, then the books of later scholars are recommended due to the clarity of their expressions. It is not obligatory to understand the books of the predecessors; rather, a Muslim is only obligated to learn the individual duties that Allah has made incumbent upon him.