You can tell that with each route they have been trying to elevate themselves and mostly succeeding. In each subsequent route they add more girls, more scenes with more variations to the dialogue, and it just comes across as more ambitious even if not completely polished. In a way it's a shame as an Azula fan that the fire nation came so early in their evolution. So far B4L is turning out to be the best chapter yet even though the memory game is super annoying.
Yes you can clearly see the pattern of them wanting to surpass themselves with every next "book" they make.
Although for my taste I don't see book 4 love route as the best one so far.
My book rank would go something like this:
Book 1 love route<book 1 slave route<book 4 slave route<book 4 love route<book 3 love route<book 2 slave route<book 3 slave route<book 2 love route
However, I'm not objective with this ranking nor do I try to be since I'm biased towards certain characters.
In short, love Azula, dislike/hate Korra (depends on her "version"), don't particularly care about Toph, can enjoy Katara being an obedient slut.
Also I think it's too soon to really judge book 4 love route.
Depending on the characters they add and the execution, book 4 love route may move up in my rank although looking at how good book 3 slave route is and especially which characters are involved in it... ngl it's going to be hard to beat.