Have you written in the 1st person in a different accent?
I have a character to a story, and I when I first started writing the story, I wrote in his Cockney accent. However, since I have written another copy though without (so I can look at them side by side). Do you think a written in accent of the whole book would add flavor, or just be too much?
I've written a few stories in accent before. It really depends on how reliant the accent is on slang or dialect. As long as you think the intended audience will be able to understand it I think it's fine to do the whole thing in accent. If the accent helps us understand more about the main character and the way he thinks and speaks than it also might be a good idea to leave it in accent.
I've done it when the character is only a bit player with a southern accent. I think it would be cool to read the story with the accent so I "feel" the character. As Titen said though, if your intended audience might get lost because they don't understand the words, perhaps you should write it without the accent and make statements occasionally about where the character is from or another character thinking the Cockney accent is sexy or something.
Another method I've used is the "interpreter". One of my novels has a story line where a computer expert is using a lot of the technical jargon (which my friend helped me with). Since I knew some of the words would make no sense to some of my readers (they confused me), I had the smart, young, computer-savvy detective explain things to her boss in layman's terms he could understand. It wasn't an exact repeat of the entire conversation, but enough to clarify things for some people.
I've written in the 1st person in a different accent--short stories. It's totally fun! Hard, though, to maintain--requires lots of editing unless you have a VERY clear sound in your head of what the accent is.
To answer your other question, writing in accent for a whole book would add flavor if done impeccably. Look at J.R.R. Tolkien, Mark Twain, Mary Shelley, Shakespeare, and thousands of others: it really can make your work stand out (in my opinion as primarily a reader and sometimes-writer of fiction).