The island of Solstice is shaped like a snowflake, with six jagged arms that protrude out into the sea on which the iceberg floats: the Sea of Moving Ice . It's a windswept place made primarily of snow and ice, presumed the Frostmaiden's "home" or a place of her power on Faerûn.
Clues
In Jarlmoot: Circle of Thrones, in response to the question to the ghostly seeress about where Auril keeps her Coldlight Roc, was the answer:
"Iskra the Coldlight Roc dwells in a lair atop Grimskalle on the mist-shrouded Isle of Solace in the Sea of Moving Ice.
Most sailors and creatures of the sea know how to find the island – and how to avoid it, if they can.”
In the revelation by the Deva of Ilmater in the chapel of the Saint Greycastle Hospital, Sudon was shown:
There is a book that in the end, the Raven Queen would like kept forever in her Fortress of Memory. The book is a holy and precious thing for Auril the Frostmaiden. There are secrets in that book. Things written down by the goddess herself - words and letters no mortal can read. But the Raven Queen is no mortal! And the Raven Queen is patient - and there is a destiny to fulfill.
The book may well help end the Everlasting Winter - the Rime of the Frostmaiden. The book may hold other lore and secrets of usefulness too.
The book is on the Isle of Solstice, which is shaped like a snowflake, floating in the Sea of Moving Ice. The book is called The Codicil of White.
On discovering the body Nass Lantomir at the Dark Duchess Shipwreck (Shipwreck, Spirits and Scion) the ghost of Nass Lantomir shared that she (guided by Mormesk, following Dzaan's own plans) she intended to bring the pirates to the Isle of Solstice to claim the book said to be capable of opening a passage through the ice to the entombed Ythryn, an enclave of The Netheril.
Results
See journal: Grimskalle.