I have done extensive searching on material on the World Tree and only found a small sample of info.
Adventurer's Vault and AV2 both have a few items "of the world tree" but no supplementary information.
Primal Power, the book in which the World Tree Guardian epic destiny was published, has this to say:
The World Tree
The most ancient primal spirit of the forest, the World Tree is closely associated with Tree Father, but they are not the same spirit. The roots of the World Tree extend to the foundation of the earth, circulating life and primal power throughout the world. In fact, some shamans say that the World Tree is the spirit of the world itself--not any particular aspect of it, but the entire creation, its interconnected inhabitants and precariously balanced environments.
Some say that Corellon planted the World Tree and in that way seeded all the forests of the primeval world. Other legends say that Corellon or Tree Father, or the god and the spirit working together, gathered seeds from the World Tree and spread them across the world to plant the first forests.
Only characters who adopt the World Tree Guardian epic destiny in this book can call on the might of this ancient and powerful spirit, as they strive to protect it--and the world itself-- from destruction and corruption.
I searched through probably 90% of published 4e books and as many magazines as I could, and this is the only information out there.
They're there for you, the DM, to create something.
The usual complaint about the Forgotten Realms is that too much is already written, making it hard or impossible to learn the setting for DMs, let alone the poor players, or for DMs to fit in anything of their own.
Having "blank lands" like this scattered around the setting leaves room for DMs' own creativity. If you have an idea, but can't fit it into one of the more popular playing regions because the lore and canon conflict with what you want to create, these blank lands are there to give your idea a home.
As a point of history, Sembia (south of the Dalelands) was a large blank area right in the middle of one of the most heavily-developed regions in the Realms, in the original AD&D 1e Forgotten Realms Campaign Set. It was set aside for DMs so that they could place their own kingdom (or expand a home game into a Forgotten Realms game by locating their existing campaign kingdom there) without having to settle for a region far away from the majority of Forgotten Realms setting material.
TSR promised to never develop Sembia so that DMs' creations would not be interefered with. That lasted all of six years before Sembia was written up in the AD&D 2e Forgotten Realms Campaign Setting, and a bitter hue and cry was the result.
So thank WotC for leaving you these blank areas. They gave you a name and an evocative description to set fire to your imagination — now follow your dreams and create something awesome that your players will love.
Best Answer
Scourge of the Sword Coast includes a detailed (and well labelled) map of the area - which tells us the hills in question are called The Forlorn Hills.
(Copies of the map, either digital or poster-print, can be bought from the artist/cartographer Mike Schley. The D&D Next adventure can be bought from DMs Guild.)
Princes of the Apocalypse takes place in the Dessarin Valley, to the north of the Forlorn Hills - they appear on the southern edge of the area map.
A less detailed, but still well-labelled, map can be seen at the Forgotten Realms Wiki.
And another can be found in the pages of the free Web Enhancement download for City of Splendors: Waterdeep.