Sure, you can rotate a cabinet 90 degrees and install its back against the side of another tall cabinet stack. This is fairly common, and the cabinets are more than strong enough to hold books as you described. Just make sure the entire unit is properly screwed into wall studs to prevent any chance of tipping. Otherwise, this setup is perfectly fine.
Yes, all of our end panels are 3/4 inch thick as standard. We do not offer thin veneer skins to cover cabinet ends. Those are difficult to apply properly and require careful trimming along the edges to look correct. In addition, prefinished or sprayed veneers do not ship well because the finish tends to crack in transit.