In Adventurers League, can a character read multiple (different but with the same name) unused Tomes of Understanding? Do their effect stack?
The same kind of tome does not stack in AL
In the most recent version of the FAQ (8.2 as of writing this answer) which can be obtained here it is clarified that you can't benefit from items that increase a certain ability score more than once (on page 9):
You can only benefit from a magic item that grants a permanent benefit to a given ability score once (e.g., tome of understanding, bag of beans, etc.). This guidance is retroactive.
In a previous version of the FAQ, D&D Adventurers League FAQ, on page 9 you could see a different wording which was later changed to the one above but may help understand what this all means (emphasis mine):
Some items are destroyed or rendered nonmagical when used, but grant permanent effects (e.g. tome of understanding, luck blades). Not only do these items still count even after they’ve been used, the Adventurers League staff has interpreted the guidance prescribed in the DMG to mean that you can never benefit from more than one such item—no matter how you come by additional copies.
All of this together makes it very clear that you can only get the benefits of a Tome of understanding once at most in AL and multiple tomes will not stack since they all grant an increase to the same ability score.