Resources for learning Reiki
According to Reiki dogma, you must learn Reiki from direct hands-on transmission from a Reiki master/teacher. Part of the idea of Open Source Reiki is to experiment and discover whether or not that is dogma or truth.
One possible grain of truth is that one must have the energies transmitted to you in a way that they are impressed or branded into your aura. The attunement process indicates that there are several entranceways into the body or aura that will allow the Reiki student to become "attuned" to the energies. With this in mind, note that static materials (books, videos, pictures, photos) probably cannot transmit the Reiki symbols to the student, unless they are properly charged with targeted Reiki energies and the intent of attuning the student (hon-sha-ze-sho-nen -> no space, no time programming and the Teacher/Master's intent). A Reiki I attunement webpage was created by Rialian many years ago, and seems quite effective. Just reading a normal book about Reiki is probably insufficient, one caveat to follow.
The caveat to learning Reiki directly from a person is that we are surrounded by spirits, whether or not we can perceive them. If those spirits are Reiki Masters?, it is quite conceivable that they could determine that any person is ready and attune them to Reiki. It is part of the generally accepted Reiki teachings that one begins to perceive one's Reiki guides after attunement to Usui Reiki II. It is possible that "spontaneous" Reiki attunements occur because Reiki guides have decided that the time is right, and the student is ready.
Traditional Reiki practitioners want to know your "Reiki lineage" to know who taught you Reiki, so they can trace your knowledge back to Usui. Part of the funny dogma here is that Usui was looking to re-discover something lost a long time ago, and there's no reason that your "Reiki" lineage must trace back through Usui when Usui himself regained the knowledge from thin air. Ostensibly, however, to be real "Reiki"(TM), you must have the dogma--catch-22--otherwise you're going further back towards the source of what is currently known as "Reiki". More about that some other time.
Many times, people have attempted to become attuned to Reiki before they are really ready. At these points, Reiki seems to slip off of them and the attunement "does not take". So another factor in becoming attuned is whether you are really open and ready or not.
With these caveats in mind, the following resources can aid in learning, or becoming attuned to, Reiki: