MacTutor
The teaching of Algebra in the early stages ought to consist of a gradual generalisation of Arithmetic; in other words, Algebra ought, in the first instance to be taught as Arithmetica Universalis in the strictest sense.
Every mathematical book that is worth reading must be read "backwards and forwards," if I may use the expression. I would modify Lagrange's advice a little and say, "Go on, but often return to strengthen your faith." When you come to a hard or dreary passage, pass it over; and come back to it after you have seen its importance or found the need for it further on.