I can't tell you for sure whether the product will work for you for two reasons:
1. I don't know your hair properties or what is in the leave in beyond the fact hat it has a pH of 4.5; and
2. pH is only part of a the story. pH balanced products are great and understanding how pH affects your hair cuticle (the outer layer of your hair) is important because it will relate to your regimen in various ways. And yes, a leave in with a lower pH is a great idea because it will shut your cuticle back down reducing frizz and tangling.
If your hair is porous (which it most likely is considering it's been subjected to heat from a flat iron - unless all of that was chopped off) then the low pH will have a definite positive effect. But this is only part of the picture. If your hair is fine and porous you probably also need to fortify it with proteins. If your hair is coarse and porous then the acidity is a great way of shutting the cuticle but you probably want to use some oils to soften your hair and prevent moisture from escaping. If your hair is not very porous then the acidity might actually prevent your leave in from being thoroughly absorbed into your hair shaft because you're blocking whatever possible doors are left to it. It is recommended that people with non-porous hair open up their cuticles a bit with warm water to allow conditioner to enter before a cold water rinse / a highly acidic product.
POINT IS, I can't tell you how this product will affect your hair, beyond the fact that I can tell you that it will help shut down your cuticle. Deciding what else goes in it depends on what your hair's specific properties (and therefore needs) are. Even if you don't purchase the book, a lot of information is available on that site that can help you understand better what your hair's needs are.