When considering GED study materials, there will be many choices. My suggestion would be either Pre-GED or GED materials depending on your reading ability. Pre-GED is appropriate for individuals who are below a 9th grade reading level. GED materials are appropriate for individuals above a 9th grade reading level.
If you are unsure of your reading level, check with your local tech school or adult tutor and have your reading assessed. It's painless and should be quite inexpensive.