Difficulties with Reading and Writing are common and can often coexist with other special needs such as ADHD and Autism Spectrum Disorders. Here are a couple resources with more information on Dyslexia and Dysgraphia:
Understanding Dysgraphia offers an overview of writing difficulties and some ways to help (while it is geared primarily towards students attending traditional school, the information can be a good starting point for homeschoolers).
Books Every Parent Should Read About Teaching Kids With Dyslexia– This is a list of books to help those teaching children with dyslexia from the Homeschooling with Dyslexia website.
Navigating Learning Disabilities and the Cost of Treatment – This article explains the prevalence of many common learning disorders and costs associated with them. It deals primarily with public school education but is interesting to read.