The Library is the area where you can manage your codices, the lists of scrolls you use to duel.
Choose the type of codex, playground, competitive or chaos brawl from the left sidebar and then create, edit or delete your codices.
A codex is composed of 2 sections, the "tome" and "recall".
The tome is the base list of 50 scrolls you start with and make the bulk of a codex. It's usually a mix of fluxes (that you can have in any number) and an assortment of scrolls that try to enact a certain strategy. There's a thin balance between the number of fluxes you need and the number of domains, costs and spells you need to support.
For other scrolls besides fluxes you can only include up to 3 copies of each, unless they're unique, in which case you can only have one single scroll with the associated uniqueness keyword.
The number of copies is calculated across tome and recall.
The recall is a special list of 7 scrolls that you don't have access to during the game except in the specific phases named, accordingly, recall.
Whenever you're in a recall phase you can pick any of the remaining scrolls you have in the recall list. Whenever you pick one, for the whole duration of that game, that scroll is removed from the recall list and added to your tome list.