So, I promised to share some helpful tips from Amy's presentation, "No Sheep for You". If you haven't read her book yet,you should, it is so helpful for understanding non-wool fibers not to mention all the gorgeous wool-free patterns.
What is covered very thoroughly in her book are the five main wool-free fiber families and how each one is made and the pros and cons of each fiber etc. As well, she gives FABULOUS and very thorough advice on how to substitute a non wool fiber in a pattern that calls for wool.
Looking for a yarn with similar gauge is the obvious first place to start and of course you want to make sure that the yarn is a similar weight to the wool that is recommended. ( But you might be able to fudge on this a bit if you took a DK weight, for example and knit a double strand of it to make it more of a worsted weight.) Texture is also key -- if the wool is a toothy texture don't choose a soft silk substitute or the finished garment will just not appear the same.
Amy recommends becoming a "yarn detective" and taking your time to examine the wool called for (although if you are like me you may need to do this with gloves on or ask a store clerk for help) to see how it is constructed and then look at the yarn you plan to substitute and see how it is constructed.
If the basic construction of both yarns is similar and the weight and gauge are a close match then you should have good results in your finished product. But that's where she recommends you purchase just one ball of the yarn you have in mind and knitting a good sized swatch (she calls it a "Geeky thing") and then washing and drying and hanging the swatch so you can see how it really behaves. If it passes the test, you know you have your ideal substitute.
All of the above is GREAT advice and had I used this method myself on my last project I wouldn't have substituted Elspeth Lavold's Hempathy for Debbie Bliss Prima in my Knife Pleats Riding Jacket. Hempathy was the same weight (DK) and the gauges were similar, but had I put the two yarns side by side I would have seen that their basic construction is quite different and that the Hempathy was a much thinner DK then the Debbie Bliss Prima that was called for in the pattern. I'll know better next time.
Of course that's a lot of work, isn't it? Worth it for any substantial project but if you just want a general sort of guideline on what wool free fiber to choose for what type of knitting you want to do, here's Amy's "Quick Tips":
Cables: Rowan Calmer
Durability: Linen or Hemp
Easy Care: Berocco Comfort
Fair Isle and Intarsia: Rowan summer tweed (see the pattern in her book as an example)
Softness: Organic cottons, silk or silk blends, Berocco comfort
Stitch Definition: Look for a "tightly spun" fiber
Stretch: Sock Candy (Socks that Rock), Cotton Fixation (Cascade)
Hope that helps you -- it sure helps me. THANKS AMY -- we wool free knitters are SO grateful for all your marvelous investigative work on our behalf.