In 1986, DC Comics decided that readers no longer enjoyed the variety provided by its "Multiverse" concept and, in fact, found it confusing. To remedy the situation, it called upon Marv Wolfman to write the story that would combine five continuities/histories into one coherent timeline. The comic book series was a stunning success, becoming the first of many summer-time events, which was certainly helped by the George Perez art. Recently, I picked up this 2005 novel (I hesitate to call it a novelization, as there are many differences) and gave it a read. While it does follow the same general path as the comic series, there are numerous differences. Mostly narrated by a dead Barry Allen, aka the Flash, his point of view is used to move from scene to scene. He often provides behind the scenes help that wasn't present or apparant in the comic series.
Also, some characters that were created in the comic series were given little to no time in the novel, such as the female Doctor Light, whose origin involved the main antagonist in the comic series but who is seen or mentioned on no more than a handful of pages in the novel.
Wolfman also works in mentions of the Justice Alliance of Earth-D, characters he created well after the original series, who represented a racially integrated team of characters. Supposedly, their story was one that he had wanted to tell, but he cut it due to the length of the series.
One last major difference is what happens to certain major characters at the end of the story. In the original, Alexander Luthor takes the Superman and Lois Lane of Earth 2 and the Superboy of Earth Prime into himself where he's created a virtual paradise for them. Superboy of Earth Prime isn't even mentioned (he was probably too hard to explain using the narrative style of the book) and Superman and Lois Lane have a different destiny in the novel.
While these differences are intruiging, and help to attract a reader to the story, readers who have probably read the original story at least one if not multiple times, the style of the narrative made the book a struggle to finish. While the use of Barry Allen as a narrator was probably as good an idea as any, he spends many pages pining for his lost wife, Iris. Having been dragged from his life to help to defeat the bad guys, he was unable to return to her before his death.
Wolfman also made a strange choice to have chapters that were no more than three or so pages long. While it means that you could read the book easily and entirely in bathroom trips, it felt very jarring to have such frequent changes of scene. It also meant that not much time was spent with any given situation or character, with the exception of the Flash.
Lastly, the thing that drew me out of the book most were the large number of editorial mistakes. There were many missing letters that should have been caught by the editor during a simple read-through of the manuscript. In addition, for as researched as the story had to have been, Wolfman made many mistakes on fairly simple things. For example, he continually referred to the Psycho Pirate as being from Earth 1 (he's originally from Earth 2), and he continually misspelled Obsidian's name.
All in all, the positives in the form of the changes to the story were offset by the negatives of the writing style and editorial mistakes to make the book both mediocre and disappointing. Read it if you are a completist, such as I am, but only the most serious fan would want to in the first place.