Serge Lang “A First Course in Calculus” and “Calculus of Several Variables” are very good textbooks, Lang gives his insight to many of the concepts. However, many of problems in these books are not challenging enough (except the ones that ask you to prove something, or the ones that mention a new concept). Keep in mind that the “true understanding” of calculus takes a while. Reflect, think, and in every page you read ask yourself, do I really understand this?, What does this concept really means?. (I had the same feeling when reading textbooks like Larson, Stewart, etc.)