bystander said:
Portrait mode is more of a factor of how it is packaged, the displays themselves don't really care. However, you will definitely want to look at IPS monitors, rather than TN for this, as for reading text and good viewing angles, IPS wins out.
It would help people if you also gave a budget, and a size that you are looking at. You may like a 24" monitor, rather than smaller ones.
The pixel density and/or pitch of a 24 inch monitor is such that I can count the pixels across most text: not good. The AOC 21.5 1920 by 1080 monitor I'm using to write this is not as clear as my 14.1 monitor at 1200 by 800. A 23 inch 1920 by 1200 would probably be the minimal PPI that I could deal with visually. But until its up to PPI of 108, its likely to be a compromise. Unfortunately, to get larger PPIs, you loose screen space. The budget is $250 per monitor, but flexible as I have old tv type monitors that will allow me to purchase one new monitor at a time if need be. Thanks again for your help.