What kind of mattress would be best for reading in bed
Hi Lynnesb,
Welcome to the Mattress Forum! 
This is a question mattress designers hear frequently, and their initial response is often, “There’s a reason why we have chairs…” :lol: With that being said, I think I can offer a few points that may be of assistance to you.
A seating surface generally uses higher density polyfoams of a higher ILD (see the chart in section 4.2.2 here from the Polyurethane Foam Association as an example). Something more resilient and durable would generally be recommended, such as latex or something using true HR polyfoam (2.5 lbs density or higher and a compression modulus of 2.4 or higher) in the comfort layers, which can have a softer surface feel but gets firmer more quickly as you sink into it so it doesn’t bottom out. Also, something that uses a firmer support core (this could be springs, perhaps a knotted offset versus marshall unit, or a firmer HR polyfoam core or latex core) can help keep you from bottoming out. Overall, you’ll want something that is higher density and more durable, so that it will hold shape better for the higher specific weight concentration you’ll be making by sitting up in your mattress. Springs, microcoils, latex, HR polyfoam and even buckling column gel can all be good components to consider.
The potential problem with choosing a mattress that works best for sitting up in bed is that you can end up going astray of the main purpose of a mattress, which is to provide you proper support and comfort in the reclined position, which is how it will be used most of the time. In general, just like when selecting any mattress, make sure that the materials are durable and of high quality, and that they meet your personal comfort, firmness, and PPP (Posture and alignment, Pressure relief, and your own Personal preferences), and for sitting upright attempt to accomplish that using more resilient and higher-density materials.
There is also some good information here about seat cushioning.
I hope that helps give you some general guidelines of what to consider. If you have other more specific questions, I’ll do my best to answer them.
Phoenix