None, that book would be another piece of clutter.

I am only half joking here, buying more stuff when you want to declutter is a bit contradictory.

Do you really need a book about decluttering? If you really think you do, or if you really love books, then go buy that book, but the idea of "I want to get into X, let's buy something" is, I think, an important source of clutter.

If you haven't read them yet, Marie Kondo's books are definitely worth it.

You have to approach them with an open mind, a willingness to act, and a genuine intention to try what she's talking about.

She really helped me remove a lot of clutter and look at order from a new perspective. I can't thank her enough.

I'm sure there are many other books on the topic, but hers was enough for me.