While you cannot limit orders to prevent ordering more than some threshold, you could configure shipping to add charges for a separate parcel, so you could split orders greater than 70 lbs. into two shipments. Separate products would already be calculated to ship in separate parcel containers regardless, unless you use one of the Tube container types (which will consolidate diverse products from the same order into a single parcel for shipping but are trickier to set maximum capacities for).
When you configure a product with Shipping enabled, on the following screen for the product's shipping options, you can specify which type of packaging container would be used, and how many units of that product can ship together in that container. If that number of units is exceeded in any order, then the cost+weight of another such container would be added to shipping. Container costs and weights are configured separately in Seller Admin > Cart Shipping Settings. You would just need to figure out how many of each book product it would take to weigh 70 lbs (including the box and any padding/packaging you're shipping in), then set that as the maximum units (maybe a few less to give you a safety margin) that can ship in one container.
