No you can just book a ticket like with all the others. Always buy a ticket online. Otherwise you'll be stuck in a queue. The Sagrada stopped doing on the spot visits because the queue was getting too long and Battló did as well. Only at la pedrera (or casa Milà as it's really called) can you still buy a ticket on site. But I wouldn't. You're just wasting your time waiting while all the prebooks go ahead of you.

Battló is the best one for me by far. I love the organic shapes and the light well and the soft wood etc. Wow

Strongly recommend Casa Batllo - it's magnificent. It pulls off this trick of being slightly surreal, but yet completely cozy. There's not a single inch of it that isn't unique and interesting to look at, and yet far from being visually overwhelming, it's immensely restful. I'd move into it tomorrow.

All that said, if they still have the LED-installation at the exit - and if you're like me, and a little sensitive to visual stimuli - avoid that at all costs. It is 360° (or more? I think it's the floor and ceiling, too) of flashing lights and sound, which reduced me to a catatonic state, squatting in the corner with my eyes tight shut and my hands pressed over my ears. It was almost, but not quite (which says something for how good the house itself is), enough to ruin the experience.