Is it better to buy books of knowledge or to save money for Hajj?

If you have not performed the obligatory Hajj, then it is better to save for Hajj because it is an obligation, and an obligation is the most beloved thing by which one draws near to Allah, as the Prophet, peace and blessings be upon him, said: "My servant does not draw near to Me with anything more beloved to Me than what I have made obligatory upon him."

However, if the Hajj is voluntary, and you find a genuine need to purchase beneficial Islamic books for yourself or others due to their unavailability, then purchasing them might be better. Otherwise, save the money for Hajj, as it is a great physical and financial act of worship with immense reward. Hajj, performed according to the Shariah, is better than non-obligatory charity. But if there are needy relatives or people in distress, then giving charity to them is better. However, if both are voluntary, Hajj is better because it is a physical and financial act of worship.