Marjorie Nicolson—one of the foremost authorities on Milton—examines Milton's work, beginning with the famous Minor Poems, "L'Allegro," "II Penseroso," "Comus" (and "Arcades"), and "Lycides." She explores Milton's middle years, when he was diverted from poetry to become Latin Secretary under Oliver Cromwell. Finally, she looks at the great poems, including a book-by-book analysis of Paradise Lost and a careful reading of Milton's poetic "closet drama," Samson Agonistes.
This was indispensable to me for reading Milton. However, I do not care for the author's personal comments and conjectures, and the writing isn't polished and professional. Nonetheless, it was valuable to me.