A couple of clarification questions first.
1) Do the Characters know what the book is about?
2) Do the PCs know what they're dealing with?
3) How did they acquire it?
I ask because, firstly the Law states: Never seek knowledge and power from beyond the outer gates. I add the emphasis for this reason. Hypothetically my Ex-Warden grandfather dies and I inherit a bunch of his books... in the collection is a undeciphered tome upon the outer gates. I haven't sought the knowledge, nor do I truely HAVE the knowledge. I'd say just possessing the tome doesn't mechanically violate the Law. The white council might still lop my head off... just in case... I mean we are talking the outer gates here, better safe than sorry. But no Lawbreaker stunt. If I start translating the tome, at somepoint very early it should be apparent what I'm dealing with, at this point I'd say there's a choice, keep going and become a Lawbreaker mechanically and in the eyes of the Wardens. However if I stop at that point I'd say no Lawbreaker stunt, the white councils feelings however may vary.
Now if they know what the Tome is and sought it out with intention of acquiring it for that knowledge... Lawbreaker
If they know what the Tome is and sought it out with intention of destroying it I'd say they're doing the wardens job. Those are the... flavor reasons why I'd say... it depends.
Metagame; if your PCs don't know what they're dealing with, just picking up the tome and getting nailed with a Lawbreaker stunt seems REALLY unfair; were I playing in that game I'd be pissed. If they PCs ooc know what they're dealing with it fine as long as they knew ooc that ic consequences would result... it'd be metagaming but especially in this instance having fun trumps playing in character.
As an aside, were a GM to say to me: "After vanquishing the Warlock you find prominently displayed in his study a tome written in some strange most likely inhuman language. Out of character messing with this book might have serious character ramifications, like LAWBREAKER serious." I'd probably respond... "Ok, that sounds like that could be a fun character twist! I pick up the Tome!" but that's with full disclosure and consent, a big part of our hobby that somet.. ofttimes gets left by the wayside is communication. As a GM you shouldn't force a change upon your PC without their consent, that's especially true in a game that's all about free will and choices!
Whew... ok done ranting.