One book I requested about 3 weeks ago and the sender marked it printed and sent the day after I requested it but the book hasn't arrived yet. 

help!

If its only been 3 weeks, then you should still have 5-7 days before the book is due to you. Is it still in your Enroute Tab? There should be a date that they system will take action on it that is 26 days after the book is marked mailed. If the book is not to you by that date the system will marked it as lost and you will get your credit back. At that point you can PM and the person and find out if they actually sent it. If they did then I'd recommend waiting another week to see if the book still arrives. Media Mail can be slow and with stormy weather everywhere, mail seems to be travelling slower.

If the book goes lost (this happens after 26 days, I believe) you will get your credit back. You don't have to do anything, the system automatically refunds your credit. I would wait a few days at least to order the book again from someone else, though--sometimes books just take that long to arrive, or the person sending it might really have mailed it several days later than she marked...I hope not, but that does happen all too often.

If the book goes lost and you then receive it after it is cancelled, just go to your account page and then Transaction Archive and you can mark it received there.

Cheryl

PBS suggests that you wait 2 wks before reordering a lost book incase it shows up. 

I ended up with two books by not waiting the recommended two weeks. The book I ordered and wanted had a wishlist and by chance, I was offered another one right after the first one went lost. I didn't want to risk having the first NEVER arrive as it was coming from an APO and getting bumped to the end of the WL if I declined the second offer, so I accepted the new offer. (Didn't actively go looking for it to reorder, but was offered it again because the I went back to the top of the WL.) Fortunately I was able to repost one of the copies right away and it was snatched up right away.  Hang in there! Your book should arrive soon.

The couple of times that I've had a WL book go lost: I've put my account on hold and autorequested any other WL books where I was # 1 or 2.  Just to hold off on a possible duplicate copy. 

Give it a little more time. Media Mail can be very slow and the system gives a pretty good time allotment for a book to get to you.  I always wait an extra 10 days or so before reordering a book because I've had them show up as long a month out!  I'm currently waiting on a book that was mailed on 3/17.  It was sent PBS DC, and it only showed being scanned when mailed, though the little mail truck on the map has been parked near my area for 2 weeks! Today, I checked on it, and it was scanned again, at a post office about 30 miles from where it was mailed, so it's still sitting in some black hole in PA!  Just how it goes sometimes, that's all.