Instruction: Extract information from the supporting document to answer the given question.

Input:
Question: are the shelf pins metal or plastic? and what is the backing (is it sturdy)?
Document: Ready to assemble furniture doesn't get much better. Clear instructions that are easy to follow. I put it together by myself in about 20 minutes. It's solid and it looks great. The shelf pins for the two adjustable shelves have little plastic slip on covers to protect the finish on the shelves. That's a nice detail. Very good value, definitely recommend.
I looked for a bookshelf with some charactor and found it with this bookshelf. It is sturdy, and nice looking!
They are still going strong and no sagging shelves or anything. A few in the past had valences and the like, but screwed in (they had a channel in the middle as I recall).This one has a nice, cherry finish (the Oak ones I have do not look as nice). Many of previous ones had plastic pieces or stickers to hide screws on the outside., This one has hidden cams and dowels to hide most of the hardware.
We bought two with the intention to set them up side by side like a double bookshelf. When it arrived, it was very heavy in the box, which was well packaged with bumpers on each edge of the box as extra protection. We opened both boxes (again, we ordered 2) and found that all parts were there and accounted for (this doesn't always happen). The pieces were perfect, no scratches, dents, marks or otherwise.
We are a frequently moving military family with 4 children, so when shopping for furniture, we are looking for decent/high quality, yet inexpensive pieces. We really hate shopping for it at Walmart and Target because honestly their stuff really is crap (i.e. poor quality, flimsy, easily scratched, easily broken, cheap looking etc.).
This was a very disappointing buy- returned it after trying to put it together and it kept on falling apart halfway thru! The back is a flimsy thin piece that comes folded up, and the finish on some of the pieces seem like a very artificial film that would tear very easily. The connections would not tighten correctly so it would not stay together! I bought it to go with a nice bedroom set for our guest room, what a waste of time! Will not try any of their products again.
They are colored as such to be fairly inconspicuous. I think you would have to be looking for them.
The seam piece then looked like it belonged when you put it onto the bookcase.These fold lines are hideous. In hindsight, the back piece should have folded horizontally so that the fold lines could be hidden by the adjustable shelves. They really do mar what looks like a solid piece of furniture (despite being a kit you put together). Once you sit there and see those fold lines you realize it's a cheap knockoff.
The pieces fit together perfectly, the total time to put both bookshelves together was about 20 minutes. The bookshelf is strong, looks great and looks expensive, even though obviously the price is pretty low. I am VERY happy with this purchase and I look forward to ordering other Sauder products in the future. They make exactly what my family needs, decent/high quality cheap furniture!
The only issue I have with this bookcase is the back piece that you nail in place. Because it folds to fit in the shipping box there are two fold lines that run vertically through the back. Since the two shelves are adjustable you only nail the back piece into the top, bottom, and sides of the bookcase. Thus once you're done the two vertical fold lines are extremely visible.
Answer: The shelf pins are metal.  The shelves are 1/2" thick and quite hefty.The backing is a dense type of fiberboard ....not flimsy cardboard/poster board type.  It fits well and ample small nails included to fasten it quite securely to the frame.  Has a nice look in matching the wood tone/grain of the frame and shelves.