I don't know if this is of any interest to anyone, but I've pretty much cleared the board with the "canon" books now from Revenge of the Sith onwards, so can give you a run down of them in what they offer, how much they expand on the films/series and if they're any good.
In time-line order:
Episode 1
Episode 2
Episode 3
Tarkin
One of the better books. This covers Tarkin's rise through the ranks of the Empire, but also covers some of his history as a child. He's essentially a complete badass. It also introduces Vader as a similar character to the Vader in Rogue One. So if you want to read some more of Vader being a stompy killer, it's in here.
In terms of writing, it's also one of the better written ones. None of them are blindingly well written, but this one is one of the most readable.
Lords of the Sith
If you just want to read all about how awesome Vader is and think he needed a bigger role in Rogue One - this book has it covered. It also covers his very tenuous relationship with Palpatine and introduces and interesting dynamic between the two that foreshadows ROTJ - oooooh! We also get to see more of Palpatine kicking arse, plus it has his Royal Guards in action. So some decent stuff that expands on characters who are often little more than scenery.
Pretty simple action/adventure stuff, not a great read, but some interesting things.
A New Dawn
This is the prequel to Rebels (which everyone should be watching!!!). It's a simplistic introduction to Hera and Kanan (the two heroes of Rebels). Both back stories are a bit bland and convoluted - but nothing surprising, The main tale itself I enjoyed; it's very much like an episode of Rebels.
Book isn't particularly well written. Plot is quite simple and the characters one dimensional. The back story of Kanan is as well handled as George Lucas handled Anakin's relationship to Padme (as in awful). As a Star Wars book with unrecognisable characters, it would have been "alright I guess". Adding in Hera and Kanan didnt do much. It didn't add enough to the characters and their relationship - no shocks or anything deep. Essentially, their part could have been in a Star Wars crawl at the start of the first episode of Rebels.
Catalyst: A Rogue One Story
And speaking of opening crawls, this is the story that would have been summarised in Rogue One's crawl if it had had one. Essentially, when you see Episode 4 and then match it to Rogue One and go "wow - they really expanded that little bit!!!" this book should be the same for introducing Rogue One. However, instead you get pretty much what the opening crawl would say only in 400 pages. "Scientist and wife fall for a secret plan to produce a weapon for the Empire by scientist's old friend Krennic. Krennic puts in lots of ugly, convoluted plots to keep scientist in the dark. Krennic then tries to kill scientist's wife, single handedly seems to start the rebellion to shaft Tarkin and ultimately gets the finger by scientist - who escapes with the new rebels (queue introduction of characters in Rogue One) and becomes a farmer with wife and child."
That is actually all that happens in the book. It's really, really dull. If you're desperate to fill in any Rogue One gaps, then read it. If you can live with the plot holes and not caring about Kyber Crystals, then don't bother.
Heir to the Jedi
This one shocked me. Set a year after Episode 4, it's all about everyone's favourite whining little **** - Luke. It's good. It's solid Star Wars storytelling with Luke spending a lot of the book going "I'm a whining little ****, I must try and do better - oh, and I guess I need to learn to use the force!" It's a good little stepping stone of a story to Episode 5 and gives a nice insight into both a nascent Jedi and the much battered Rebellion.
Twilight Company
Battlefront - The Book. It's very similar to what you didn't see in the battle at the end of Rogue One. Set just before (and a little after) Episode 5, it's the story of a bunch of grunts on the ground (and in space) taking on the Empire in a bit of a lost cause. It features a nice section expanding on the Battle of Hoth (more Vader being awesome). Bloody, gritty and a good attempt at bringing less glamour and more realism to the series. Until Rogue One, it felt out of place, but it's clear that there's a need for this side of Star Wars. Very much a strong war book, so avoid if you're after adventure and typical heroes - but one of the better novels IMO.
Aftermath
Set a year after the end of the Episode 6 (it starts with all the Ewoks dying in a flaming pit, burned alive by all other races in the galaxy as an act of unrepentant genocide), this trilogy covers the retreat of the Empire's forces, and them trying to gather their forces together to fight back against the New Republic. It covers a little of the "heroes", introduces some new ones and gives you a good idea of the infighting and betrayals in the disintegrating Empire. There's a little that covers the struggle of the New Republic forming too.
Overall not very well written, with a bit of a dull story.
Aftermath: Life Debt
This is better though. The second in the trilogy it expands on the characters a little (badly though at times), but most importantly, it brings us back to Han, Chewie and Leia and what they're up to. It's good to get back with these characters and the ending of the book is pretty good. Still not very well written, but it's getting there.
Bloodline
This is set about 7 years (apparently) before Episode 7. It's a great story for Leia's character and it's really nice to see her given a bit more of a role in the universe. The story is mostly just politics though, with a crap sabotage/mystery sub-plot to try and liven up the senatorial power plays and bickering. Dull, dull, dull. It does set up some background to Episode 7, and I'm sure that the gaps between the years will be filled in by Episode 8 and 9. It gives a reason for the Resistance to be a thing and also The First Order. I assume the third Aftermath book will link the Aftermath series to this.
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Just something that is worth noting across the books until Bloodline. Right through from the early books to the end of Life Debt, there are a lot of recurring characters (both Imperial and Rebel) that crop up as antagonists and heroes again and again. It's really nice to have these characters flow between all the books - it makes a connection through them all.
Something else that's quite interesting is the way that the fall of the Jedi/Republic is handled and the rise (and fall) of the Empire. Suddenly, things start to make sense in the universe we see in Episode 4 and the way that the technology seems to have changed, the reason for smugglers/Hutt power, why the Rebellion grew so strong. They fill in a lot of background across the universe that gives the films additional gravitas. None of the books offer a greater insight into individual films (they all stand or fall on their own), but it's interesting to see the machinations that make the stories in the films happen.
In terms of preference, I'd say if you're going to read them, read them in order. If you're going to cherry pick, then pick the ones with the characters you're interested in, but in terms of quality I'd put them:
Tarkin, A Newl Dawn, Lords of the Sith, Heir to the Jedi, Twilight Company, Aftermath/Aftermath: Life Debt, Bloodline, Catalyst.
There are several "Young Reader" novels that I haven't read yet, but will get around to (no Kindle versions, will have to hunt down physical books). They cover the battle of Jakku and some additional snippets of Han/Luke history between films.
Knowing
@Howie isn't going to read any of the above, I've also made my way through these for him:
They contain lots of pictures, words with 2 or fewer syllables and an Ewok at least every 5 pages. Definitely recommended for the ROTJ lovers out there!