https://www.ef.com/english-resources/english-grammar/past-perfect-continuous/ https://www.englishpage.com/verbpage/pastperfectcontinuous.html https://www.thoughtco.com/past-perfect-and-past-perfect-continuous-1211193 "She had been going to the library before she went home." This would sound better as, "She went to the library before she went home," so it does not make sense to use the past-perfect here. Past-perfect is not best for repetitive actions per se. They are best when describing an action that was occurring when something else happened. "She had been researching the topic at the library, when the librarian suggested another source." "She had been thinking about going to the library, when two of her friends invited her to go to the library with them." "He had been studying for a long time, when his mother suggested he take a break."

Your sentence is a bit off. Personally, I would only use "go" in past perfect continuous for repeated actions, not continuous, unless I am using a rarer definition or a phrasal construction. I looked at the strange, glowing book. I *had been going* to the library every afternoon for the entire summer, but I had never noticed this book until now, the day before the beginning of school. Sam and Jill *had been going steady* (=dating) for three years when Jill decided to break up. My neighbors were fighting again. The fight *had been going on* (=happening) for the last hour, and there was no sign that it would stop any time soon. The wind-up toy was still going (=operating). It *had been going* for the last hour without being re-wound. ****** The example you gave might work better as future in the past in the past. Future in the past: he said he would come. OR he said he was going to come. Future perfect in the past: Standing in the trench, he realized that, by the time he returned home from the war, the apple trees would have already blossomed. Future in the past in the past: I had been going to go to the library before I decided to stay home instead.

'Going to the library' can be either of two things: Going repetitively, as you said 'She was going to the library every day'. It can also mean travelling: She was going to the library when she had her accident. In your case, it sounds like the second version, but it doesn't quite make sense given the tense you want to use. That's assuming you are trying to 'use a tense' rather than convey a meaning. In past perfect continuous, you need to have an event that changes the state. You could say "She had been going to the library when she realised she was late, and went home instead." Or, in the continuous case, you need an example that makes more sense: She had been going to the library every day until she got her job.

Void!