Right now, I am just taking pictures of books. I want the camera for the best still pictures there are. I went on Sigma's site and saw that pictures there were significantly better than what my camera produces. I want that same quality.
I have a 18-55mm and a 55-250mm. Both Canon. I am pleased with both. I am not a specialist in cameras though.
Well we know there are a number of factors that affect the quality of a picture besides the camera, such as lenses, camera supports, lighting, post-production, and sheer technique.
It can be argued that comparable IQ can be delivered by all current Canon FF cameras under a wide range of conditions, so you can consider the Canon 5D mkiii and even the 6D if you're mostly taking pictures of static objects.
If you go into a FF camera, one thing that will need to upgrade is your lenses. Your EF-S lenses won't work on any Canon FF camera, and better lenses may be needed to deliver top IQ anyway. As you upgrade consider the EF 24-70mm mkii, a 70-200mm tele to replace the 55-250mm, and perhaps some primes regarding your needs.
Are you serious that those two lenses will not work on the DX1?
But considering you will primarily be shooting books and you say you have the money for a 1DX, it would be a better idea to buy a 5DIII (half the cost of a 1DX) and use the difference to upgrade your lenses.
Then, in a year or two, you will have a better understanding of your needs and what you may or may not be lacking from that set up. At that time, the question of the rumored high MP body will be answered and we'll know more about where Canon is heading regarding studio/landscape vs. sports/pj bodies...including the 1DX replacement.
I shoot with a 1DX and I love it. But I am mainly shooting moving subjects often in dim, ever changing light. I need the speed and the high ISO capabilities. This body has given me the ability to use one body instead of two. I used to shoot with the 1DIV & 1DsIII. The 1DsIII was used mainly for performer headshots. At this point I am satisfied that the 1DX images for headshots are as good as the 1DsIII's. The slight diff in MP doesn't matter in my case (I really need to sell the 2 cams I'm no longer using), however I will likely be very interested in the high MP body.
If you purchase a 5DIII and a few good L lenses, when the next gen bodies are announced/released you will know whether it makes sense for you to upgrade.