I’m not really sure what you’ve got going on there… I don’t think it’s gimbal lock, if it was, switching to quaternion rotations would cure it. Changing the order of euler rotations should stop it as well, but neither of those work. I think it’s a problem with the limit rotation constraint. Since you only want the cover to rotate in the y, I locked the x&z rots in the transform panel and unchecked them in the constraint. The cover now rotates only in the direction you want it to, but you still can’t limit it’s rotation so it only opens so far.
Looking for an answer, I removed the constraint and added a single bone to control the cover. I got the same results, I couldn’t limit the rotation of the bone with a constraint so the cover would only open so far.
To help make things a little worse, the file doesn’t work right in 2.49 either. I opened the file in 2.49, switched the 3d manipulator to rot, and I was able to rot the cover on the x-axis as well as the y-zxis with the manipulator. So the constraint isn’t doing it’s job there either. Same thing happens as far as locking down the x&z rots and limiting only the y rot so the cover can only open so far. It can’t be done, as far as I can tell.
Personally, I would just lock the x&z rot in the transform panel, get rid of the constraint, and leave it up to the animator to not open or close the cover past where it should rotate. I mean, if it’s just you animating, then it’s no problem, you know not to open the book too far. If it’s someone else, then it’s just one of those things they have to control - Just like a character, it’s up to the animator to not allow it’s feet to go through the floor, or the hands to not go through an object.
Sorry I couldn’t be of more help, if someone else has an answer, I’d really like to hear it as well…