FINAL FANTASY X-2.5～永遠の代償～ The Price of Eternity

The story of Spira, following Final Fantasy X-2, is spun into a novel! Set two years and several months after Yuna defeated Sin, Spira has transitioned into a new era. Yuna's plan to unwind with Tidus, cherishing their long-awaited reunion, took an unforeseen turn when a storm battered their ship, casting them ashore on unfamiliar land. Removed from Besaid's vicinity, Yuna found herself confronting the hidden truths of this newfound island... —Japanese-to-English Back Cover Synopsis (~Eien no Daishō~, 2013)

A fan restoration project, entitled Rebirth ~ The Price of Eternity, started by Danko Kaji of ffx3chat.

Fan translation source here. Continuation of Rebirth by mishyoona here (ongoing).

Quick review: The portrayal of Yuna, as well as her relationship with Tidus, was (for the most part) lacking. I would still recommend it, but keep your expectations low when delving in.

Go in and just have fun with it.

I will say, the book excels in world-building and lore. I'd suggest exploring the marked chapters for those parts, at the very least!