En ou le?
Are these equivalent:
J'en ai lu.
Je l'ai lu.
If not, when exactly to use each?
Le (it/him), La (it/her) and L' (it/him/her) are Les (them) = Direct Object Pronouns (COD). They are used to replace the noun. For example:
John lit le livre >> John le lit (use le because livre is masculine). It makes sense to use when you know what noun is being replaced.
As-tu lu la lettre? Oui, je l'ai lu
The pronoun 'en' has many uses including the expression of quantity
Elle a trop de livres (she has too many books) >> Elle en a trop (she has too many of them) Elle a dix livres (she has ten books) >> Elle en a dix (she has ten of them) J’ai quelques livres (I have several/a few books >> J’en ai quelques-uns.
Are you trying to say that you have read the book? If you are trying to say you have read a lot of books you could say << Des livres, j'en ai lu beaucoup >>