I recently bowed to temptation and began buying the non-standard Bob's Red Mill flours. I started out with just rye and semolina (I've only begun working with flour that isn't ordinary bread flour) and yesterday I just bought a package of barley and spelt, sorely tempted by the flax meal and amaranth. I'm not looking for recipes (although they are appreciated, of course), rather I'm curious about how each different type of grain affects the flavor. For example, I found that semolina makes for a wonderful crust, but too much and the crumb gets a bit blah. Likewise, good use of rye imparts a fantastic richness and deepness to the taste, but done poorly leaves an unpleasant "hole" in the flavor.
One I'm particularly interested in is tapioca. One of my friends used it to make some truly fantastic rolls (Fogo de Chao's recipe) and I'm quite interested--and intimidated--by the gooey texture.