The message of the longest memory was cruelty of slavery, and thus - racial inequality. The novel displays the effects of slavery over two generations, starting all the way back when they arrived from Africa. The Africans were seen as inferior, and were therefore treated in some of the least humane ways possible.
This is displayed through characters such as Chapel.
Chapel, like his Father, had no christian name. Although he was named after Old Whitechapel, this was not considered a christian name, as Old Whitechapel was named after his owner - Mr. Whitechapel. Although he was perfectly capable, he wasn't allowed to read or write. When he was caught by Mr. Whitechapel reading with Lydia, he was whipped with a belt and told "do not let me catch you reading again. If you do you will be sent away, far away to a place where slave boys die of hunger, hard work and the whip." This could almost be seen as a cruel sense of irony, as Chapel does end up dying due to the whip.
Cook, doesn't have a christian name either, it is even doubtable that she has a name at all, as when she is first talked about by Sanders Snr, she is simply referred to as the "slave girl", and eventually becomes Cook. Cook was raped by Sanders Snr and although the action itself isn't necessarily racist, although it's wrong, the only reason why it can be considered racist is Sanders Snrs reaction to doing such a thing, as he feels no remorse towards Cook, and feels that...