I have a two part novel. I searched Lulu site and did not see any information on "sets." So, if that's not a marketing and pricing option yet, then perhaps it can be. Vol. 1 and Vol. 2 can be sold separatly at their respective single book price. But, have a special price to sell Vol. 1 and Vol. 2 together at a special price. A set might require a new bar code and ISBN. Or, an option could be available that both books can go into the "cart" at a slightly reduced price. Example Book 1 is $20 and so is Book 2, (same topic or story of course). Combined the "set" would be $36. In my case "Hawk Dancer" is book one. Soon to be publish is "Cloudburst" the follow up novel of the same story.