That is just a massive number of books. Very nice.

The general rule of thumb is to use books when you need cooldowns decreased or chance to land debuffs increased. That's where you get the maximum RoI.

From your list, Rhazin seems like a good choice if you plan to use him for CB. You want his A2 booked so the cooldown is on-par with CB rotation. Hurndig also makes for a great arena nuker, and can benefit from books for dungeons.

I haven't quite decided where to use them, but I'm leaning towards making Rhazin a CB champ, so that I can replace Serris and Athel with Rhazin and someone else with atk down (potentially Jareg to get a def up buff for free). For the rest I'm more focused on dungeons than the arena at the moment (though I'm even buying tokens right now to take advantage of the "new segments" to farm medals while it lasts), and I can't really afford to dedicate champions to the arena exclusively. Candraphon does alright as my arena nuker meanwhile.

Pretty accurate assessment. Definitely want rhazin booked as you said with a3 optional. Scyl and drex great without books. Definitely want to make the cb replacements you are planning too.

The ones I have and can comment on: 

- Sethallia: My first lego, so fully booked her, kind of by default. Even though she's amazing, with that powerful heal on a two turn cooldown as well as a good AOE buff strip, I have to admit I don't use her that much anymore. BL Faction Wars, certain situations in Doom Tower (she's especially good against the Celestial Griffin as she can solo him up to a certain point, or probably even the top ones if you give her crazy enough stats, but also on the Eternal Dragon, Nether Spider, Frost Spider,...) and 3v3 arena - definitely still some use, but nowhere in dungeons, CB or regular arena, now that I have Arbiter. 

- Scyl: Had her for ages, never booked her. Her use became very limited when I got Arbiter anyway, though now in DT there are a few fights where she's better than Arbi (or where it makes sense to use both together). Booking her or not mostly depends on how badly you need that AOE stun - if she's largely a dungeon carry for her heals and revives, as she was for me, she doesn't need books. 

- Drexthar: For a while I couldn't figure out what was so great about him or why he showed up so much in team recommendations for DT (hard DT, anyway). But once I got his passive fully booked and got to the tough fights where HP burn becomes increasingly effective compared to direct damage, I started to appreciate him more. For now I haven't continued to fully book his A2, though I might - almost entirely for DT, though. 

- Rhazin: Fully booked him, mostly use him in CB - that's mostly just the A2 indeed, the A3 and A1 are more for arena, if you use him there, or some DT fights. Though the A1 does help on non-void affinity CBs who buff themselves. 

- Hurndig: Only starting to build him - no books for now, might consider him in the future but right now my books are going to Astralon still. 

I should add, for Drexthar, if you mainly use him as weak affinity tank on Spider (I don't, I have Kantra for that, though I might consider a switch for Spider 25 when I get there), I agree he probably doesn't need books, since the spiderlings would constantly target him even without any provoke and they'd hit him often enough that even his passive wouldn't need boosting. But in DT, you do kinda need the passive boosted. 

I might book Drexthar at some point when/if when I start using him elsewhere, but for now I feel I have more pressing priorities. I realize Sethallia will probably be left behind at some point, but she's relatively cheap and safe to book and right now I sort of need her kit. I'm struggling hard with spider, so her plus Hurndig (who can also do wave AoE on other bosses) are looking like attractive options.

Drex doesn't need books for spider or DT, what level spider u trying to beat?

At this point, I can auto 13 and usually manual 14 if I don't get really unlucky. Auto 14 is possible as well, but fails more often than not. I've got Paragon, so I might be able to manual past the bad affinity stages, but I don't have the control and/or damage output to get away with it, especially if the spiderlings get ideas to target anyone else.

My spider team right now consists of Athel, Candraphon, Serris, Coldheart and Apothecary. I'm aiming to replace Apothecary with Sethallia for the added cleanse, debuff block, and turn meter control (HP burn won't hurt either). This is mainly why I'm looking to book her in the first place. I'm considering removing Serris and adding in Hurndig, and possibly replacing either Athel or Candraphon with my 2nd Coldheart once I have her built up. I'd lose the atk debuff by removing Serris, but I'd get more damage and turn meter control. I could also stick an Armiger in there for added turn meter control and max HP damage scaling. I may have to swap in Serris depending on affinity if Hurndig can't get the job done. If I can get away with using Paragon to tank manually until 16 and maybe 17 and then 20 somewhere down the road, I can use Drexthar as the weak affinity tank and hopefully be able to auto it with the turn meter control from Coldheart(s), Hurndig, and/or Armiger.

With 2ch and armiger big spider should never get a turn. drex for burn and sethalia should work thru 20. drop armiger against blue, replace with scyl.  you may have other options i am not aware of too.  athel definitely drop, candraphon too.  hurndig i haven't built yet but obviously looks perfect with right team.

Oh I'm dropping both Athel and Candraphon, it's just a question of which one first. Hurndig has just about everything to combat spider: AoE def down/weaken/acc down/HP burn, a 100% turn meter reduction, AoE nukes. It just hurts my soul a bit when I think of the number of books I'd have to spend on him to get him to full potential.

At some point I'd like to try a team with Drexthar, Sethallia, 2x Coldhearts and Hurndig. 3x 100% turn meter reductions on a 4 turn CD should ensure spider never gets a turn if I can time them right, HP burns all around, def/acc debuffs as well as weaken, possibly even provoke and atk down if Drexthar can hit his debuffs, with a pretty strong heal and backup turn meter control from Sethallia. Only thing missing would be a def up or revive to make sure Drexthar doesn't bite it at an inopportune time.

Hurndig  HP  Burn  seems  so  situational.

It is situational, but with the spider spawns spread out, so will their turn meters be, so there's a good chance it'll proc on 2-4 spiderlings. It's not the main attraction here though, it's really just an added bonus to the turn meter reduction and weaken debuff, and in this scenario Drexthar would be the main source of HP burn.

A huge number of books, that's great.

Closing this Necro. It is very cool how mnay books you had a couple months ago Edgnit! :D