They will absolutely accept it. Never once have I had them turn odd packages away from me that I am checking in as luggage. That's the upside, they're going to take it. The downside, however is that they don't do much to "treat" your package for you - so you've got to make sure you pack it well enough so that it doesn't come apart during the baggage handling process. I've checked some poorly packed boxes which, in hindsight, I absolutely shouldn't have checked and got very lucky that they didn't come apart during baggage handling (one of my boxes came apart completely as I lifted it off the luggage carousel in HKG, another two were completely destroyed but the goods still recoverable off the baggage carousel in SFO). At check-in, however CX didn't say anything...they just slapped baggage tags on and let me go. I feel pretty lucky and don't check any more brown boxes except wine cases anymore (see below).

I still regularly check cases of wine in normal-looking brown cardboard boxes (with Styrofoam inserts that fit the wine very snugly in the box), but those boxes are specifically made for shipping wine. Both FedEx and UPS accept them as proper ways to ship wine, so even though they look like normal boxes they're really quite secure.

Lastly, if your package eats it in flight and you don't end up with your books at the end, I'm not sure if you'll be able to get compensation (and at the least, it'll suck not having your books).