Sir Richard C. Jebb, Commentary on Sophocles: Electra
Old-fashioned but influential commentary on the play, available here online for free. Might be interesting to compare and contrast with your own ideas about the play, or indeed, the ideas contained in the Summary and Analysis sections of this ClassicNote.
Introduction to Sophocles' Electra (Haverford College)
http://www.haverford.edu/classics/activities/electra/electra.html
A good online resources base aimed at Classics students. Well worth reading, and reasonably regularly updated. Good background information on several issues associated with the Greek theatre.