Well I'll just tease the first couple of abilities (scanning my copy of the book was the best idea ever):
BATTLERAGER ARMOR
When you choose this path at 3rd level, you gain the ability to use spiked armor (see the "Spiked Armor" sidebar) as a weapon.
While you are wearing spiked armor and are raging, you can use a bonus action to make one melee weapon attack with your armor spikes against a target within 5 feet of you. If the attack hits, the spikes deal ld4 piercing damage. You use your Strength modifier for the attack and damage rolls.
Additionally, when you use the Attack action to grapple a creature, the target takes 3 piercing damage if your grapple check succeeds.
RECKLESS ABANDON
Beginning at 6th level, when you use Reckless Attack while raging, you also gain temporary hit points equal to your Constitution modifier (minimum of 1). They vanish if any of them are left when your rage ends.
Ah screw it, why not a third:
BATTLERAGER CHARGE
Beginning at 10th level, you can take the Dash action as a bonus action while you are raging.
Pretty cool stuff. I can see that this would be a fun class to be. I'm not sure about it being the best grappler, however. Grapplers excel at locking down opponents and controlling the battlefield. The more you take your focus off of that target and move it onto other things, such as dealing damage, the less effective you will be at your primary focus. Nothing wrong with being moderately good at two things instead of very good at one thing, but it's generally not the way to make the most potent build.
Here are some specific pros and cons:
Dwarves Only
This build is specifically for Dwarves, which isn't so bad. Each of the 3 dwarf types makes a decent grappler, so you can certainly make it work. Mountain Dwarf, one of the best grappling races, conflicts a bit. You can start with 17 Strength, so you want to take a Feat at level 4 that gives 1 Strength. But the best feat in that category is Tavern Brawler, which gives a bonus action grapple, which conflicts with the bonus action spiky rattle. A minor quibble but it's there.
Battlerager Armor
Three free damage every time you grapple is probably the coolest selling point here. It's only 3 damage, but it's still pretty cool. The bonus action attack might be a fun attack against a grappled and proned opponent, but that bonus action will likely be busy (especially if you MC into Rogue for Expertise and Cunning Action).
Reckless Abondon
This is largely useless to a grappler, sadly. Attacking recklessly is just not something a grappler should generally be doing; it doesn't give advantage on grapple checks. You generally only want to attack when you have the opponent on the ground, which gives advantage anyway.
Battlerager Charge
A great ability if you go straight Barb or if you only dip 1 level for Rogue. Mobility is king when grappling. But once again, that bonus action is getting very crowded. If you'd gone with Totem Barbarian, you could have gotten +15 to movement when raging, which doesn't cost any extra actions.
Overall I'd rate it on par with the other two barbarian builds. A great build for a damage-dealing barbarian grappler, particularly if you're only taking 3, 4, or 5 levels in barbarian.