In addition to the books already mentioned: when I was a student, I
benefited very much from reading August Cornu's Marx and Engels from
Liberalism to Communism ( in Italian). I think it was a shortened
version of a longer French version (3 volls.), but I do not know if
there is an english translation. The book was excellent  as an
intellectual history. I do not think that it is a good idea to
separate Marx's  economics from his philosophy, and an understanding
of, say, his 1844 economic and philosophic manuscripts may be very
useful. Cornu's book was very clear and helpful in many respects.

Lilia Costabile