I personally don't. I have articles saved in pocket, and I do have textbooks in PDF form in GoodReader that I use in split screen mode - but for long form leisure time reading I do go back to my kindle paperwhite. I've always wanted a light form factor and, while I would like a larger screen size, I'm prepared to sacrifice that for the e ink screen.
I occasionally read non-fiction books on my Air 2. But find the backlight and weight was too much honestly.
U can do it but it's a little big and heavy. I use kindle app on my 12.9. U get used to the size. It feels small to me now. But if u have no reason for the 12.9 size I would say smaller is better. I love the 12.9 for art creation, photo editing , and as a second battery powered screen hooked up to my cintiq companion. It's a great text book for school, but reading in bed is a little uncomfortable
I read books on mine all the time- iBooks and kindle. I am embarrassed to say that I had to change auto-lock from 2 minutes to 5 minutes, sometimes the larger screen would darken before I was done reading both pages.
Other than that it's fine for reading.
I read on mine every day, everything from developer documents, PDFs, novels in iBooks and Marvel Unlimited. They're all awesome on the big screen.
I use it in portrait and landscape, for portrait I started putting my hand across to the furthest away side of the iPad and I rest it on my forearm, rather than with my old ones I held it by the edges. I find it very comfortable that way.
I've still got smaller iPads, other tablets and my trusty Kobo H2O, but nothing beats the big screen of the Pro.
I am reading magazines and it's great on the 12.9 iPP! Never did that on the 9.7 iPads because they where too small. The 12.9 has the size of a real magazine page and it's even better than the real thing....
I read a lot in bed and at times lay on my side with my iPad Air locked in landscape mode resting on it's side. Not sure I could easily do that with a 12.9 - if I decide on the bigger one I may have to get a Kindle.
It is about the size of most hardcover books when they are opened and layed flat. I see no problem wielding something that size. Not sure how thick of books you guys read but if it's a hardcover the weight isn't too far off after holding it for extended reading sessions. I know holding iPP for reading my textbooks instead of the book is a LOT more feasible than my large volumes.
In all, reading is one of the most important and useful features of the 12.9iPP imo.