I made a book that I was trying to rig. It's just a front piece (cover and half the pages) and a back piece (back cover and half the pages). The problem is as I open it, the cover tilts some

As the images show, the bottom splits a little and the top doesn't. In actuality, the 2 parts cross a little. The whole row of vertices were in line originally. How do I fix this?

you have a few options.

open the Joint editor in Poser, change the Orientation angle slightly while the book is open. Depending on what dial is opening the book you have to use either X Y or Z. make sure the axis ( Green ) is on top where it is not torn apart else it will cause that both sides split ( Move the axis to the top corner ) .

second option as it seems that you use the bend off feature for this articulation would be to merge the edges L + R of your book can be done in Blender or other 3d program in this case you would rig it bendable, a little more work.

I don't know if this is relevant but 3Dcheapskate created a simple book figure for Poser that does do what you are attempting.  It can be found here:

Proof Of Principle - Poser ERC Opening Book - Poser - ShareCG

I know you would rather figure this out yourself, however, it might give you better insight in how to approach the problem.

Some months back, the forum gurus kindly explained that Poser morphs are linear, so the 'hinged' item I used as morph target would fare badly beyond small-ish angles.

Needed a different approach...

Okay, finally had a chance to get back to this. what DownTown37 said helped. I needed to adjust the zrot of the orientation of the bones. It seems to have done the trick. Thanks all for your suggestions

I just started a own little test for a poser book using a bendable unimesh "simple skinning" rigging witch is very simple and offering you amazing results, at least better then  with the bend off option in Poser or traditional poser rigging that could cause issues with the XYZ axis. You just need to Group the movable parts, The more the better, Morphs are not needed anymore having way better options with articulations. XYZ dials getting to be your best friends when rigging it unimesh.

Wow... very nice book!

Thank you Y-Phil

The 4 pages have a secondary layer this allows you to write your text in white on a black paper using these in the material room as trans and diffuse using your own colors or make them glow. a really easy setup to add your own made textures for the pages. Now I will separate the material zones for each page this way each can get it's individual texture on the layer, the book cover will get a mechanical magical seal and also a secondary layer for decorations. I was also thinking of making one page high res to be able to add morphs like a face coming out or water effects but I have it triangulated and would have to make em solid quads at least for the page that would get morphs. Alternative add a conforming page to the book for such a feature.   

to get the rounded pages when opening, the book needs to be made in a closed stadium. not a easy task to group, after rigging when opening you get this rounded page effects, so yes it can be closed into zero pose and all is straight, just like real. you only get this realistic effect this way. The traditional way of rigging in poser would not allow this effects as the pages are overlapping when the book is closed simple skinning unimesh in poser makes it possible and the only thing that needs to be done is to smooth out the bending with the joint editor paint tool on the individual groups of the pages.  

Now each page has its own additional material layer with a minimal distance to the book pages so each can use it's own map or coloring like in the case shown. A book like this gives you amazing possibilities :)

That is absolutely amazing.