This would be the one tool I would NEED. I love my Joshua Roth Hori Hori (means "dig dig") Japanese gardening knife. I can dig, weed, lever, pry, and cut with it.The picture above is the model I have, but there are a number available online. Just be sure to get a carbon steel or stainless steel blade, and you're probably best off if you get one that's made in Japan. They sell a cheapo one for $5.99 at Marshalls, but I know from experience that it rusts and the handle breaks off. So, expect to pay between $25-35 for a good one and consider it money very well spent. I can even spear slugs with it, but my slug travails are a story for another day.
Also, I highly recommend wrapping a bright piece of duct tape around the bottom of the handle so that you can find your hori hori when you put it doen in a pile of leaves.
If I could take one more thing it would be the Reader's Digest Illustrated Guide to Gardening. As a neophyte gardener I have found it to be indispensable. I'm sure it's not the best book out there, but it covers almost every plant and situation you can imagine without totally terrifying a less than expert horticulturist, which goes a long way in my opinion.