I've poured over all of the rule books and I can't find any rules for recovering ability damage save Restoration. Can someone help point me in the right direction please?

I think recovering from ability damage is all self contained in the damaging effect. It is pretty rare.

It is hidden in XGtE under the downtime rules: Relaxation

While relaxing, a character gains advantage on saving throws to recover from long-acting diseases and poisons. In addition, at the end of the week, a character can end one effect that keeps the character from regaining hit points, or can restore one ability score that has been reduced to less than its normal value. This benefit cannot be used if the harmful effect was caused by a spell or some other magical effect with an ongoing duration.

thanks for the feedback, I looked at the rules as per the Xanathar's and Intellect Devourers are a hell of a lot scarier now my colleague will be out of action for 1 week in-game as she rests and recuperates. We have no access to the high-level spells needed for an immediate fix. Cheers all

It's not that rare depending on what you are talking about. Intellect Devourer, Shadow, Wight, Oblex, Demilich, Death Kiss, and several other drain ability scores. 

Quote from MagnusPale >>

It's not that rare depending on what you are talking about. Intellect Devourer, Shadow, Wight, Oblex, Demilich, Death Kiss, and several other drain ability scores. 

 

 

 
So maybe a dozen out of several hundred; seems rare to me. It’s something they very much dialed back for 5th edition.

True.  Also, each individual creature lists the rules for regaining the stat. Shadows strength loss returns completely after a single short (or long) rest, regardless of how much strength was lost.

In general, that is the common rule - these things are supposed to be abilities that drain for the length of the battle, not some super hard thing to cure.

It's already a more powerful attack than going after HP - lots of high CR monsters have a low stat that is easy to lower to the point you beat them.

Note Shadow 2024 rules eliminates that ability score return.  Shadows are now extremely dangerous.

This actually just came up in a game I ran yesterday.  In 2024 the rule is found in the Rules Glossary under "Long Rest".

"Ability Scores Restored. If any of your ability scores were reduced, they return to normal."

Yeah I dug around after posting that and found that as well.  Really dumb that it isn't called out in the Monster Manual, SOMEWHERE.  

Some people had the same doubt, so it's something understandable. E.g.:

- Monster Manual 2024: Missing text from Shadow? Is it connected to a rules change for short/long rests?
- Life Drain duration

This new way of explaining and understanding the rules seems intentional, as noted in some pre-release videos. For example, New 2024 Monster Manual | Everything You Need to Know (starting at 00:30:21)

[...] the monster manual also in many ways is a culmination of things that people saw in the new Player's Handbook and the new Dungeon Master's Guide because the Monster Manual relies on many of the rules enhancements in the Player's Handbook, it relies on the thinking we had in the Dungeon Master's Guide about making DMs lives easier, and so you get to the Monster Manual and this is all about taking rules elements from the Player's Handbook mixing it with our "make DMs life easier philosophy" from the DMG to make this host of monsters that are there to inspire, to be fun at the table, to surprise players, to feature elements of the rules that are new in the new versions of the core books like there are more monsters now that make use of the fact that the game has the term Bloodied back so you're going to find monsters that have things triggering within their stat blocks when they're Bloodied, you're going to see Exhaustion used in ways that it hasn't been used before because the condition was redesigned and now we can use it in certain ways that we were hesitant to use in the past [...] the monster manual truly is the culmination of the core book revision that we've done in part to simply celebrate 50 years of D&D and now you get to see how all three of these books work in unison now [...] you're also going to see many of the innovations that were originally presented in the Player's Handbook really take form here [...]

Or New Constructs, Plants, Beasts & Humanoids (starting at 00:19:05):

[...] it feels complete now, it feels like I have everything that I need to really start playing this to dive in and experience all of the best parts of the new rule system, really so much of what's in the Monster Manual is using what had its groundwork laid in the Player's Handbook, in the DMG, now that you're seeing all of these great new rules actually being utilized in the monsters in play at your game table [...]

Quote from Khanthiilas >>

I've poured over all of the rule books and I can't find any rules for recovering ability damage save Restoration. Can someone help point me in the right direction please?

If you mean ability drain type of effects. They come back after a Long rest:

Ability Scores Restored. If any of your ability
scores were reduced, they return to normal.

On principle, I think the shift is a good one. The issue is just that people are used to seeing the clear conditions in the effect, so with those preconceptions the change throws people off. One of those little speedbumps of a rules update.