Check out the following resources for information on harvesting yarn!
1. For everything you need to know and more, check out:
2. On finding the right fiber items to harvest:
3. On the harvesting process:
4. Useful info on yarn weights, etc. (I look to these all the time):
5. Even more resources can be found on the Ravelry "Thrifty Knitters" group page!
I share my own harvesting process on the DIY page here.
Some inspiring people and sites in the fiber arts and sewist communities:
Notebook by Jared Flood of Brooklyn Tweed