You've gotten the memo—do your pull-ups for a bigger back. Let's revise that a little bit: Those pull-ups you've been doing have given you a wider back, but what about adding some thickness, detail and (most importantly) functionality to your new muscle? The "back valley" will add a third dimension to your back, which along with your "wings" will make this important muscle group more complete.
Deadlifts are the king of back development, period! There are several deadlift variations including standard, sumo, rack and deficit. I recommend at least trying them all at some point in your training. Form will always be similar and there are three things to always remember:
1. Keep your weight in your heels.
2. Keep your chest high and back flat (no rounding!).
3. Pull back! Start light and increase weight only when you feel comfortable with form. Four sets of 8-10 reps works well.