First, some thoughts on books, some of which may be self-evident. The usual progression for learning from books starts with libri, because the 1/3 Art limit means that most of them are low-level anyway, moves on to summae, and finishes with tractati, because anyone can study a tractatus, regardless of their Art level. Once your Art is equal to half that of the best mage in the world in that Art, you're pretty much done with summae.
I bring this up because this sequence suggests that we may want to deprioritize tractati; we can always trade with Lemannus for them later, when we've used up all the libri and summae that we can find or write for each other.
It also suggests that when possible, people should try to plan their book learning to save the summae until you've exhausted the libri. In the short term, we should try to assign the summae to the mages who are going to be writing the putative low-level libri.
I also revisited the munchkin analysis method I mentioned earlier, because I realized it didn't account for multiple seasons of summa study. Upon the revisit, summae came out extremely well. It's a pity most of them are too expensive for us.
Anyway, the books that interest Marcus most are:
Creo
Summa, lvl 8, Q10 [26]
Liber Questionum, lvl 3, Q12 [15]
Liber Questionum, lvl 8, Q14 [22]
[I can't make ideal use of the lvl.8 liber right now, but it would be just right after the lvl. 3 liber or the summa. I'm most excited about the summa, though; it's a substantially better value.]
Muto
Liber Questionum, lvl 3, Q15 [18]
Rego [71]
Liber Questionum, lvl 10, Q12 [22]
[I really want this.]
Aquam
Summa, lvl. 10, Q8 [28]
[I really want this too.]
Herbam
Summa, lvl 12, Q10 [34]
Imaginem
Summa, lvl 10, Q11 [31]
[These summae I don't care about so much, but they're good investments.]
Mentem
Tractatus, Q4 (Aura of Rightful Authority) [12]
[For the reasons discussed above, I think we should try to trade for this at a later date. But I would like to lay hands on it eventually.]
Terram
Liber Questionum, lvl 3, Q15 [18]
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I could make good use of the Animal and Corpus lvl. 0 libri, but I gather that we have magi who could write those, and it isn't pressing.
Both the low-level Rego libri are a waste of time; I can write better. I cannot write a lvl. 0 Auram liber yet, but one more level and I'm good to go, if no one else has substantial Auram. I suspect our member yet to appear in Polybius' account may.