After reading your post and the replies to it, it made me think that Lizzy is emphasizing the motive for her reading. I don't like to think she was being falsely humble (a la Bingley). Not only, as others have said, does she sprinkle her conversation with literary references, she is often reading while at Netherfield. So, maybe she says, "I am not a great reader," meaning, "I don't read to be considered accomplished, to impress others, or to teach them. I read for enjoyment and enrichment." In other words, she reads for herself, not for others.