Do the Manuals and Tomes allow you to exceed 20 in an ability score? And can they be stacked?

Question

I am looking at items like Manual of Bodily Health, Tome of Clear Thought, etc.

Quoting from Manual of Bodily Health

If you spend 48 hours ... your Constitution score increases by 2, as does your maximum for that score.

This sounds to me like when you read the tome according to the instructions, you are then allowed to increase your CON up to a maximum of 22.

1. Is that a correct reading -- that these magic items allow a player to override the usual ability score maximum of 20?
2. Probably, you were already at 20 when you read it, but if, say, you were at 18 and read it to bring your CON up to 20, could you then later increase your CON further via feats or ability score increases gained at certain levels?
3. Can you read more than one book of the same type, to increase your maximum by an additional 2 points?

Answer 1

Yes. This is correct.

To answer you bullet for bullet

1. Yes. This is what these do.

2. Yes, this is a correct reading, if you go from 18 to 20, you can still increase to 22.

3. Yes*, most things in 5e stack, though sometimes things don't stack with themselves (this isn't a bonus though, it's an increase, it's different). However, this is probably rare enough that this should not be an issue in a campaign (an if a DM gives out more than one of these per PC it's their own fault if their game breaks because of it). *I don't have this item's text in front of me so this is barring a caveat in the description, which if it existed would likely preclude this particular bullet anyways.

So basically, yes. This is an item (among several and a L20 class feature or two) that breaks the normal rule for maximum 20 ability scores. It breaks it, and it does so permanently for the character.