What books to read?

All of them, basically.

There are 5(?) novels but my favorites are The Masked Empire and Asunder, which take place just before DAI. But if DAO is your favorite game, The Stolen Throne (which is about Maric and Loghain), The Calling (more Maric and Loghain plus Grey Wardens), and Last Flight (which is also about Grey Wardens, and goes back and forth between the events of the Fourth Blight and right around the time of DAI) might be of interest to you.

There are two big lore books, World of Thedas and World of Thedas 2, which are excellent. The Inquisition art book is also pretty meaty.

There are three different comics series. I liked Knight Errant, which takes place during Inquisition. Magekiller was kind of meh, and you've already read the other one.

Oooh, those covers. 😍

Knight Errant doesn't take place during DAI. It takes place after the Trespasser DLC. Magekiller is the one that takes place during DAI. 

I kind of figured, but just felt like that should be noted since I wouldn't be surprised if that series taking place post-Trespasser may mean that Vaea and Aaron are among the "people he doesn't know" that the Inquisitor talks about at the end of Trespasser. 

Nunzio DeFilippis and Mike Laidlaw said that it takes place after the Exalted Council but still the same year as Trespasser, so basically shortly after. Remember at the end of Trespasser no matter what you choose there is still a secret Inquisition that exists and is operating to stop Solas. That's the scene where the Inquisitor and a few others meet in the basement. Charter is Leliana's successor as spymaster. 

The big problem with KE being set after Trespasser is that there was a codex in the DLC that says Meredith's statue was moved. People in Lowtown even made up a song about it.

But I can see why the shadow Inquisition would be involved in the events of Knight Errant. After all ...

Yeah, but I figure that’s a replica. It ain’t glowing and shit.

Just because Marius and Tessa worked for the Inquisition before Trespasser doesn't mean that they would be fired. They are well-trusted members and there is also the personal relationship between Tessa and Charter, that even being joked by Varric as the reason Charter asked Vaea to rescue them, so it makes sense they would stay if they so chose. Yes the Inquisitor will be getting new people as well, but they still keep those they trust.

Yes, I'm sure if Vaea and Aaron show up in DA4 there will be a bit of story about how they got involved including the events of Knight Errant. It's possible that those two may even be our starting companions, as well as a mage so we have one of each class like in DAI. 

Why are you putting everything in spoiler tags? Everything we are talking about came out years ago.

No matter whether you make the old Inquisition a part of the Chantry or dissolve it, by the end of Trespasser there is always still a new Inquisition. We see that in the final cutscene as well as even the choice about what to do with the of Inquisition. Keeping the old one gives you more influence but has a bigger risk of enemy infiltration, while dissolving it gives you less influence but makes enemy infiltration more difficult.

Regarding Sutherland, their mercenary group doesn't always exist since that is a player choice. If the player didn't make Sutherland's company, Tessa and Marius do that part with other Inquisition soldiers. Same with the part with the Bull's Chargers since they could be dead by the time you go to the Hissing Wastes.

As for why Charter goes with Tessa and Marius, we see in Magekiller they are very skilled at infiltration for example when we see them kill several of the Venatori leaders. So they are a good choice for that kind of job, just this time it didn't work out so the Inquisition needed someone even better, hence then seeking out Vaea. I agree Charter uses questionable methods but really what can you expect from her given her past as The Black Hart as well as Leliana, who also used questionable means as spymaster, being her mentor.

I hope that as well. I think Bioware learned their lesson from DAI with that looking at MEA. In MEA while there was a book about the Nexus Uprising the game still gave lots of information for it. 

Because the original poster was asking about which books to read, which I assume means they don't know these things and would rather I kept them hidden. Rather than take the chance of annoying someone for revealing too much, I prefer to annoy someone by using spoilers.

They were good for the job assassinating leading Venatori because they were dangerous mages, so in theory would know how to neutralise them. It was Archon Radonis' idea to use them for that job specifically because they were outsiders and so likely not to be known by their targets so easier for them to get close enough to take them out.

That is not the same skill set as is needed for the tasks set for Vaea. I just question the fact that those other two were thought the best choice out of the available personnel in the Inquisition if many of the most trusted and efficient operatives had been retained. Then it is decided to use a complete outsider for such a sensitive mission before her loyalty to the organisation had been proven. That suggests that the majority of trusted personnel had been retired from service after the Exalted Council but Tessa and Marius had been retained. Why?

The Bible.