I've found that pickling agents and scraper are a really ugly matter (and probably unhealthy even though nowadays it says "bio" on the label) and success is, well, mildly impressive. It's more impressive what drops of that nasty crap do to anything you don't want pickled (including tools and clothes).
The plane surely does the job (tried that once) but it's a bit like trimming your lawn with a flamethrower. Sure enough, it's a clean surface afterwards, but it's quite abrasive. If there is any kind of ornament (e.g. a bevelled or rounded edge) you will lose detail, and if there is a verneer over the edges (assuming it's not massive wood) you may damage that. Also, on any kind of plywood (wood core or otherwise) you can easily damage the surface beyond repair by cutting away the topmost layer of veneer.
Now to the actual question, is it safe for the plane? It will sure need some cleaning, but with most paints (especially old ones), I would seriously doubt that it will do any damage to the plane. There certainly exist paints which are about as hard as glass, but if you have one of these, you will know (you can tell immediately by looking at it). It's unlikely that you'll find one of the super-hard paints on some inherited bookcase.
Paint (old paint in particular) may of course contain metals (above all, lead), but hard wood is much harder than lead, so if that was a problem, you shouldn't plane any common hardwood like e.g. oak either.
On the other hand, I found that simply using 60-grain sandpaper (and later grinding "up" to 120 and 180) works wonders on old paint. Don't even need an electric tool, a plain old manual sanding block and going with the grain does it really, really good (and with a pleasing outcome).
Also, with sand paper you can go into the edges without taking the whole bookcase apart. It's a bit of fiddling, but you can do it. With a plane, you must take everything apart, there's no alternative.