A reading specialist, at the core, is a reading teacher. His basic competency is to have the knowledge and the skills required to teach a child how to read. He can teach a child to identify and sound out letters, put these sounds together to say a word, link that spoken word to meaning, read and comprehend longer texts, write thoughts, express feelings and ideas, etc. A reading teacher should be able to do all these things. A reading specialist can do more.
A reading specialist knows when reading breaks down for a child. She can tell when this would happen and why. She can pinpoint what is missing in terms of knowledge, skills, and attitudes. More importantly, she knows how to prevent the break down long before it occurs. But for those who are already having a hard time, the reading specialist help by teaching the child what he needs to know. The reading specialist can show how things in reading are done right so the child can watch and learn. She can make what is often too obvious to notice, even for the expert reading teacher, a delightful surprise for the struggling reader.
The reading specialist can also help the highly proficient reader. He can further enrich the reading experience by making recommendations for books to read, authors to sample, websites to explore, etc.
A reading specialist cannot be a speech therapist, an occupational therapist, or even a development pediatrician or neuropsychologist. For a child to learn how to read, that child needs to be taught. No one can do this for a child but a teacher.