Are there folks who are "naturals"? Probably, but I've not met any. Could they benefit from an attunement by a Master? Most likely. Can anybody learn this from a book? Reiki practitioners say no, I don't believe so, and I've not seen anything in this thread to suggest otherwise.
Actually, there are people who are naturals. They are naturals at directing and manipulating energy in non-reiki related ways. This was me before I met Master Blacston.
Before I met him, I was...ok...at directing energy at muscle injuries to help unkink knots and the like. Not great, but good enough that people came back to me time and again.
It was at an SCA event where Master Blacston walked past me and saw/felt what I was doing. He said that I had a talent that could potentially be directed and refined if I wanted to learn Reiki.
I said that I did and thus I was taught and awakened. Afterwards I learned that what I was doing before was the energy version of working a knotted muscle with massager wand. Not all that great, but slightly more beneficial than just the physical manipulations.
Afterwards, I was able to do more...much more.
And it is possible to learn from the books, but you can not do it from just the books alone. The really good books or websites out there tell you that these are only exercises that you can do before you get attuned, or things you can to do to strengthen and improve your skills at your level. They cannot raise you to the next level, but they can guide you at your levels.
The dodgy books out there are the ones that say "You can learn all that you need to know by reading this book."
Those are the books that are either teaching you another (and equally valid) form of healing, or are like all the books that teach some of the higher-level networking protocols and certifications without you having to ever touch a computer.