I often buy multiple books on trips and carry them home. For the outward bound trip, the suitcase is 10-15 lbs (4.5–7 kg) or more below the maximum weight limit. That way I know I have some room for souvenirs and books. I've also taken older clothes that I'm willing to toss out at the end of a trip.
If I think there's a chance that a bag is overweight, I may put one or two of the heaviest books in my backpack.
I usually carry on a backpack, not a suitcase, so I have space constraints. I'd much rather make it as light as possible, so I pack the suitcase to be checked as heavy as I can.
If you're taking a suitcase on board that won't fit under the seat, remember that you're going to have to lift it up in the overhead bin.