Tome Requirements / Dependency?
You have to unlock 2 tomes from the previous tier to get access to the next tier. To research a new tome you have to research 3 (or maybe 4?) spells from your current tome.
Tier 1-3 tomes do not have requirements. So as soon as you can research the appropriate tier you can select any of these tomes.
Tier 4 and 5 tomes do have affinity requirements. They are all single affinity tomes, so you only need to gain points in the correct affinity to get a specific tier 4 or 5 tome.