All Knowledge Is Contain In Livejournal - food query

When I was at Uwajimaya shopping yesterday, I picked up an interesting-looking root vegetable on a whim. It's quite large -- maybe a foot long or possibly even larger and too thick around to circle with one hand although small enough to easily circle with two hands. It has tan skin with evenly but irregularly-spaced darker spots, and out of each darker spot comes a few thin fibers like hairs. The flesh is smooth and just off-white (sort of cream-colored), with no appreciable odor. The label for the vegetable called it a [Something] Potato, and there might have been another, non-Anglicized name. Unfortunately, I did not write down what it was, and I have since forgotten.

It's definitely not a lotus root (no holes), and I'm pretty confident based on size, shape and appearance that it's not a regular potato either. It's also not jicima. It doesn't look like any sweet potato I've ever met, but that doesn't mean it isn't. My best bet right now is that it's tapioca root -- does anyone have any experience with tapioca root to confirm or deny?

Any ideas?

(Uwajimaya has more Japanese food than any other food, but it's pan-Asian, so this isn't necessarily a Japanese vegetable.)

EDIT: paperclippy has solved the riddle -- it's a yamaimo, or mountain potato. Yay!

Food: San Shoku Domburi (Three-Colored Rice)

A few weeks ago I saw rowr talk about doing the Washoku Warriors challenge -- basically a group of people who agree to make recipes from Washoku: Recipes from the Japanese Home Kitchen. The cookbook appears to be a primer on Japanese traditional cooking, with an emphasis on home cooking -- the kind of thing people would make an eat for themselves (as opposed to restaurant or inn food, or fancy cuisine). It looked like a lot of fun, so I ordered the book and decided to give it a try with the first dish: San Shoku Domburi, or Three-Colored Rice.

One side note. Normally, I'm a very, um, relaxed cook. I adjust, substitute, throw things around, do what seems right. Normally this works great. However, if what I'm doing is trying to learn another culture's cuisine, I make the recipes by the book -- at least the first time. That's because I want as much as possible to try to taste the way it's 'supposed' to taste. Then can adjust to suit my tastes, once I have some idea what it's meant to be like. I also try not to make substitutions unless I absolutely can't find the ingredient (and with Uwajimaya down the street, that doesn't seem terribly likely).

Since I'm going to be doing -- hopefully -- many recipes from this one book, it feels a bit unfair to the author to publicly post the recipes themselves. But I am going to blog the process I went through to make it, so you can probably recreate it if you want (and if you're really curious about a particular ratio or something, feel free to ask).

( More, including pictures, behind the cut )