I am trying to find a Star Trek book where a battle is occurring. The battle is so big that it looks like the sun. The battle occurs at the end of the book
The Voyager novel Ragnarok is about two races locked in combat - their weapons were underdeveloped compared to their shield technology, so the battle consisted of millions of ships pressing against one another until a ship's shields failed and it was crushed.
Voyager approached, mistaking it for a strange star based on the energy output, I believe.
After trying to determine the cause of the battle, of which both sides do not remember how it started, Voyager somehow gets involved (naturally) and finds a way to shift the frequency of their phasers to slice through the ships, destroying many on both sides of the conflict.
In the end, both sides agree to a cease fire to fight their new enemy, the Federation. The only problem is that their top speed was warp 1 or 2, so they won't be in Federation space for a thousand years.
Are you talking about the novelization of Star Trek: Insurrection?
Chapter 13, page 279:
Picard closed his eyes, but for a blazing milli-second, it was like staring wide-eyed at a daytime sun...then the brightness dimmed....
It's not really that big of a battle though; this was just a struggle between Picard and Ru'afo.