What are the obligatory and recommended matters that a Muslim should learn in the lands of the disbelievers, and what books can be used for that purpose?

Seeking knowledge is an obligation upon every Muslim. Religious sciences are of two types: obligatory (fard) and recommended (mustahabb). The obligatory knowledge is that which rectifies one's belief, worship, and profession. Thus, it is obligatory to learn the six pillars of faith in general, and to know what validates ritual purity (tahara), prayer (salat), and fasting (sawm). It is also obligatory to learn the rulings of lawful (halal) and unlawful (haram) in the profession one practices. As for other sciences, they become recommended, unless specific duties become incumbent upon the Muslim, in which case it becomes obligatory for him to learn their rulings, such as Hajj, Zakat, trade, marriage, and divorce.