Want to restore a Harry Bertoia slat bench  

It looks
like ash to me.
Other than the chip at the lowest slat, a thorough sanding should get rid of the water stains. The chip can be filled. . .

the
wood is maple. you ask the value. if you are going to resell it you would want to think long and hard before altering the finish. pieces often bring as much or more with some condition issues, than having been redone.

And yet another opinion regar...
And yet another opinion regarding the species of wood -- it's definitely not maple. It could be ash. But I think it's more likely oak. The woodgrain is consistent with oak and it's got a traditional oak stain finish. Also check out the picture I attached from a similar bench sold at Treadway. The description lists it as an oak slat bench...I say it's oak, and I'd be reticent to alter it if the only damage is what you pictured.
Value is high hundreds, under a thousand.
http://www.treadwaygallery.com/ONLINECATALOGS/Sept2004/1001-1050.html

Maybe it's just your photo, b...
Maybe it's just your photo, but the oak appears to have already been sanded. The edges, particulaly at the end, seem overly clean/sharp too, as if recently sawn.

Grass...you're so lucky to have one
because the Bertoia bench is a hellava expensive bench...more than the Florence Knoll and way more than the George Nelson.
It's worth whatever you have to pay to restore it, you lucky dog!

You're right on sanding
It has been sanded because the guy I bought it from said he sanded it lol. That is why I have no idea what the finish was. The underside still has its sticker though so he either took it off and put it back on or didn't do anything to the underside...