I hear a lot that you should not train pull ups to failure, so I usually do strength training with lots of rest in between sets. The reps depend on whether I choose to do Fighter pull up mode (5-4-3-2-1) or SSPT mode (8x2), but always try to do same or higher volume than before.
However, would it be better if on certain days where I may choose only to do pull ups, I train in steps (1-2-3-2-1..) and do more volume. Yesterday I did 48 reps as compared to 16 reps I do usually. Occasionally I failed on the max-rep set with a 60-90 second interval between sets.
My pull up progression is the worst in my overall bodyweight training. One of my concerns is that I am extremely skinny, and I think if I only do strength training, then perhaps my hypertrophy gains won't be as high as my strength gains, and I won't have a big enough fuel tank to progress well.