One thing I can tell you about the user's manual is that you should check the manufacturer's website and see if you can download one for free. I've done this with many things I've gotten at yard sales, thrift stores, etc.
As for how to use it, mine has been extremely useful at Thanksgiving and Christmas. I only have one oven and, as you know, meals at these holidays usually involve multiple side dishes. All that have to be cooked in the oven.
Last Christmas, I put a green bean casserole, a sweet potato casserole and some rolls in my roaster. I separated the casseroles in layers, using small wire racks, and cooked all of them together.
Worked like a charm.
You can use the roaster like a "giant" crock-pot and cook a large quantity of chili, stew, soup and then serve or freeze as you wish.
It is also wonderful for roasting a large chicken, with veggies, or a turkey.
Once you get your instruction booklet, you will get a better idea of its capabilities.
There are others on this site who have experience with this appliance, too. They will give you additional guidance.
"As a girl I had zero interest in the stove." - Julia Child
This is real inspiration. Look what Julia became!