It seems that the every other book in the Thomas Boston collection is eligible for the Logos Now Free Book of the Month, except Volume 6.
Is this typical? That the entire set, except one volume qualifies for the free book coupon?
Can't say for sure, but you could try putting the one volume in the cart and using the code anyway to see if it works.
The new Faithlife Books store has a lot of problems. The particular set I'm completing doesn't even show up in the list of eligible books, but if I put it in my cart on logos.com, the code works with it.
I sent an email to Customer Support and heard back almost immediately — volume 6 was accidentally excluded. They fixed the problem and it's now eligible for the promotion.
Thanks Logos!