I am not sure if the problem persists at your end still. In any case I will share my experience so far. My dog is 13 months old and has always had this problem of howling and chewing up books, TV remotes, slippers, shoes, clips, etc. when left alone for even 20 minutes. This continued even after his teething phase. We both are working and my parents are staying with us and there has not been even one single day since his puppy days when he has been left alone in the house for even half an hour. We actually used to avoid going out together because of this problem. I have always attributed this to anxiety problem. However I went through all possible postings on the net and read up books on dog psychology, etc. to figure out how to deal with this. I suppose it did help me in a tremendous way. This time around when my parents were gone for 2 weeks and we had to leave him alone for 3-4 hours at a stretch (we go home for lunch for an hour) for 7-8 hours everyday. We took some measures and it did show some very good results:
1. We used to leave our worn clothes to go for laundry wash scattered around the house before leaving. Dogs see/feel with their noses literally. If you leave your scent scattered around the house they are less likely to feel insecure when you are not there.
2. Leave chewers partially hidden around the house for them to smell and hunt for those, an exercise basically to keep them busy and not get bored.
3. Stuff some toys with dog food and leave them around in different spots for them to again figure out how to get those out of the toy, etc. again one exercise to keep them busy.
4. Keep all precious, chewable stuff out of reach as much as possible.
5. and of course, walk him for half an hour twice a day daily, and make him run/ chase him around the house(as a game) for 15 minutes daily to basically exercise him and exhaust him of the extra pent up energy.
Well.....these few steps have so far been successful in keeping my dog from chewing up my house this time. Hope it helps you too!!!