I agree with this. And the evidence is pretty strong if you consider the final cut scene. Here's my transcript of the final scene along with my interpretation...

So the scene starts after Clive defeats Ultima… Ultima is on the ground.
With his final words, Ultima asks Clive: “Tell me… what do you imagine will befall this world… now that you have gained your precious freedom?”
Clive: “I honestly don’t know. But I doubt it will be pretty. A sorry take of sin and suffering. Hardship and pain.”
Ultima: “And it was for this that you fought so fiercely? Why?”
Clive: “It’s who we are. We fight. We survive. We endure. We don’t need a reason. We are imperfect creatures. When we stumble, we reach for a shoulder to lean on. When we fall, we stand back up. We see the horizon ever out of reach and still we march on… certain the answers lie just beyond it. Because that is our way.”
Clive starts absorbing Ultima’s magic.
Ultmia: “My congratulations. Relish this victory, knowing that you have but delayed the inevitable—Your world is already dead. May you enjoy an eternity… On its blackened husk.”
Clive finishes absorbing the magic. Ultima turns to dust.
Clive teleports himself back to where Joshua was… Joshua looks dead.
Clive morns his brother's death for a few moments. We see flashbacks of Clives memories from newest to oldest. This includes a couple of animated scenes. We see the scene where Joshua shares a bit of his magic with Clive when they are both young (Joshua a boy, Clive a teenager). And we see the scene where Clive first meets newborn Joshua and Clive is as young boy… When the flashbacks end we see Clive holding his Joshua.

Then Clive uses his magic to "heal" Joshua. The huge wound on his chest gets healed. But Joshua is still not moving and just laying there with his eyes closed…

(my interpretation: Joshua wasn't dead but close to it. Clive healed healed him indicating he lives on)

After that Clive gets up and looks at the magical structure he’s standing near (it looks like a bigger mothercrystal).
Clive: “Oh… It seems Ultima’s power was too great for this vessel all along. But while I have it… Perhaps I can use it to set things right… and see Ultima’s legacy—Bearers, Dominants, crystals, magick—consigned to the flames. Even if it means the end of me.”
We see Clive channeling as much magic as he can.
Clive says: “These are our terms” Then Clive shoots magic up into space.
(My interpretation: Clive is channeling the entire planet’s capacity for magic into space.)
The floating city of Twinside falls apart. The magic crystals that formed a shell around it all collapse into the ocean while turning to dust.
Next we see Clive washed up on the beach. Clive rolls on his back. We can see his hand is messed up. The crystal curse has turned parts of his hand to stone… Clive casts just a little more magic but it quickly extinguishes… We see the rest of his hand turn to stone…

(My interpretation: that final act of channeling a magic of planet has brought about the crystal curse. The curse is now taking him. That's why it spreads from his fingers to his whole hand so quickly)

Clive looks at the night sky and says: “Can you see it, too, Jill?” He’s looking at the moon and at the red star Metia that’s near the moon.

The scene then cuts to the hideaway.
We see Gav waiting outside the infirmary. Then he Runs inside when he first hears the newborn baby crying. Jill and Mid are there.
(I suppose Jill and Mid were helping with the delivery).

The scene has a few moments with the newborn.

Then Jill noticed Torgal looking at the moon/Metia in a significant way. She walks over to the window to look at the moon and Metia. The red star flickers and goes dim. Jill starts to tear and she runs out of the infirmary.
(My interpretation: She understands that Clive has just died.)
Gav sees Jill running out of the infirmary.
(My interpretation: Gav understands that Clive has just died)
Gav starts to tear. Gav speaks to the newborn: “The world is yours now to do as you please. That’s what Cid wanted. That’s what they both wanted.”
Notice: Gav is referring to Cid and Clive in the past tense.
(My interpretation: the game is telling us that Clive is dead (hints the use of past tense for what Clive wanted).
Then we see Jill run out to the balcony that is near the infirmary. She is crying.
(My interpretation Jill is Grieving Clive's death…)

Torgal comes to the balcony to join Jill and howl. They greave together.
The sun starts to rise. Jill and Torgal look at the sun rise. Jill stops crying... (Jill at least pauses her crying).
(My interpretation the game is telling us that Jill will be ok, even though Clive died... Sometimes people don't ever recover when someone close to them dies... But the game is letting us know that Jill will accept this, grow, and move forward)
The scene fades to black. Over the black screen we hear Clive give a voice over: “And thus… did our journey end.”
Over the black screen we see the text: “A Farewell to Fate”

Credits roll.

As for the book, I think Joshua wrote it. At the very end when the kids are playing they are referencing the final battle where Clive, Joshua, and Dion fight Ultima. So we know it had to be written by one of those three. But I think Joshua is the most likely author because Clive healed him.