I will now discuss the plots of the Books in LOK and some of the worldly themes.
Book 1: Air - I think Air is the most complete Book in the whole series. The main reason for this is that it wasn't predicted that LOK would go past a season. As such, he has a clear rising action, falling action and climax to carry it through. At the same time, it makes it hard to rate because it is equally better and worse than multiple Books because of its resolution. However, the strengths in this season outweigh the weaknesses. For instance, Amon and his Equalist propaganda is one of the best villain agendas I've seen in any medium (being voiced by Steve Blum helps too). Along with this, the world or city-building done in the first season does a great job in showing how much has changed since the ATLA and who all of these characters connections to the earlier heroes are. Where this season falters the most is its romance plot, which is hackneyed, hard to pull off with teenagers and just distracts from the overall plot and themes. Still, this is a great introduction to the world of Korra that connects with the world of ATLA while growing its unique sense as well.
Book 2: Spirits - Often described as the weakest Book in the series and I'm inclined to agree. The romance plot continues, Korra is still hotheaded, and Unalaq is a fairly weak villain. Furthermore, it seemed that the creators weren't sure where to go next from Jinora, Aang's kids, Mako, the awesome new character Varrick, Bolin and Asami's plots, along with the spiritual world. As such, I think it had issues finding a focus as it didn't give enough time exploring the Spirit World. However, The Beginnings Two-Parter with Avatar Wan was brilliant and many will think it's one of the best episodes of the whole series (ATLA included) due to how much backstory it gives along with its animation. Lastly, Spirits has some great moments, even a Kaiju battle, but its lack of focus brings down the overall season and has my lowest ranking for the Books.
Book 3: Change - I love Change. It is hands-down the best Book of LOK and in my opinion the second best Book in the series after Earth -at times I think its better too. There are two large reasons why Change works: One is that the Beifong family crisis is wonderful character development, especially for Lin; Two is that Zaheer and the Red Lotus are fantastic benders and offer some of the best & most creative fights in the entire series. As such, Change does such a great job alternating between villains and heroes while deepening the characters without silly romance plots on both sides. Also, the conclusion of the Book is devastating to Korra and a real bittersweet note as Jinora comes into her own. I want to say more, but like Earth I feel that experiencing the more open world nature of Change is a treat for yourself to experience.
Book 4: Balance - Balance is another hard Book to rate because it goes along a different perspective than the previous books as it is far more introspective for Korra and many of the characters as well. As such, while it doesn't have the creative fighting of Book 3 until the end, it does a great job of deconstructing what the entire series even means as the characters are in such different positions. I personally like Balance, with its standout episodes like Operation Beifong & Korra Alone, plus it finishes the whole series on a strong note as well. On the other hand, I think it's biggest flaws come from the lack of action seen in the first half of the episodes except for Korra Alone. Still, Balance is a solid Book and resolution to the franchise.
Worldly Themes: While ATLA had several themes to explore, I think LOK has more mature and adult themes than its predecessor.
Change the Status Quo - I think one of the larger themes of LOK is turning everything that ATLA set-up on its head. Each of the villains have noble and logical goals that place what we know about the world in a state of chaos. Amon is against bending, Unalaq wants the spirits to come back (which in turn makes the world more magical in a sense), Zaheer wants anarchy, while Kuvira wants for the Earth Kingdom to be stabilized and possibly more control overall. Furthermore, the Fire Nation is not the villains at all, airbenders return, Republic City represents unity amongst the benders, and the deconstruction of the Avatar (loss of bending, no hero, loss of connections to past lives and more) takes place. As such, it offers a more mature viewing of the world of avatar in which the characters are no longer little kids.
Feminism - This theme is one of the largest themes in the entire series as even ATLA had independent and badass ladies (Toph, Azula, Katara's development). However, in LOK it takes precedence it it's final scene (I'm just going to say bisexual love), how undeniably strong the women are - Kuvira, Lin, Korra, Asami, Opal, Jinora, Su, P'Li, Ming Hua, Zhu Li and even basic soldiers -, its constant push to have women as the saviors while men are weak. Also, the entire series and especially the final one is essentially a rape allegory in my view. First, Korra has her mind damaged, then her spirit and body, which eventually breaks her mentally, physically and emotionally which is a rape situation. Along with this, much of Korra's struggle in Book 4 is PTSD from a traumatic event that could be linked to rape. While this wouldn't be blatant in a kids' show, many of the ideologies are present from my perspective. Most importantly she survives, copes and along with the rest of the female cast becomes stronger than any hate.
Responsibility - This theme ties into the next theme, but much of LOK deals with predetermined responsibilities either by family or duty. For example, Lin's responsibility as a police chief, Mako has to deal with basic police duties in Book 2, Korra as the Avatar, Tenzin as a master, Kuvira to her country, and many others having responsibility to their families. However, much of this show deals with moments in which characters have to assume and reject responsibility for their own terms (Korra becoming spiritual = assume, Bolin leaving the Earth Empire = reject) in what leads to fairly grey territory.
Family Issues - I personally like how much this alters the characters of ATLA significantly. While Mako and Bolin learn a lot when they visit Ba Sing Se and Korra has a lot of issues with her uncle & her father in Book 2, some of the best family issues come from direct family members of the original Gaang. We see that Aang wasn't the best father in his admiration for only Tenzin and lack of responsibility to Bumi and Kya. In addition, we see that Toph's lineage has a lot of issues stemming from her encouragement of their freedom. The whole family unrest that occurs in Book 2 with Aang's family and Toph's daughters, Lin and Su, is some of the best character development found in the whole series as it deals with no just an adolescent, but an adult issue of dealing with your siblings & parents who were not as great as you thought. This is also an issue amongst the villains too from Amon & Tarrlok, Unalaq and his brother Tonraq and Kuvira's abandonment of her surrogate mother, Su. As such, it creates an uneasy dilemma that not only shows the Gaang in a new light, but also is a relatable problem that many families face.
All in all, Korra and her Krew is a great successor to Aang and his Gaang. Thus, I highly recommend watching all 7 Books from this glorious series and world that the creators have built. Both shows demonstrate what can happen when a Saturday Morning cartoon reachers another level.