Hi!
  I have a couple of talented artist friends, each has offered to do a little work for my book. I created a simple design doc with a wish list of my pics and the basic info about what would be in them.
  My questions:1)  is there an industry standard for this type of thing? 2) What is the best level of detail and 3) What is absolutely required info?
  Thanks for your attention,

There really isn't a standard. I've done everything from following very detailed descriptions to working with no description and the game text only. This is what I'd suggest:

Give them the text that the art would most likely be appearing with. The section that they're illustrating. Or give them the entire game's text and just reference the important section(s). Also give them a brief description of what you're looking for. Once you've done that, they will either get right to work or they will ask you questions until they're ready to begin. Most importantly, just have good open communication with them to make sure that you're giving them everything they need.

That make sense?

In my experience, you get better work if you keep it general and let the artist do what they're good at, i.e., be creative. You should certainly spell out any specific requirements. This can be detail specific to your game world. Knowing nothing about your game, e.g., if your elves have a lot body piercings make sure to mention that. Or it can be specific to your production needs, e.g., dimensions, color palette, orientation, direction of the flow of action, etc.

And, oh yeah, listen to Jennifer. She knows of what she speaks.

Reference shots are very good things to have if you need all of your art to be similar in some sort of way.  I've got reference art being done up right now (which will then be forwarded to Jennifer, among others) so that the artists can all have their naval officers in the same uniforms and the two alien races looking the same from one piece to the next.

Some artists will benefit by reading the text they're going to accompany; some will not (some will ignore it entirely).  First off, ask them what sort of support material they would like to see.

Secondly, I find it useful to focus on strong, active images.  Instead of saying "please illustrate something to accompany this segment of text" which might get you anything in response, saying "please illustrate two naval officers firing down a corridor at fleeing pirates" will get you what you actually want and need, and there's still a good deal of latitude for the artist to still have a little fun with it.

To reiterate a point that might not be obvious, strong, active images are important.  Art is what gets your potential customers to open the book and identify with something that they want to do.  If they open the book and find a scenic landscape shot, there's nothing for them to do.  Additionally, keeping an eye on your gender and racial mix can't hurt, either -- if your customers are supposed to identify with what they see and all you display are aryan ubermench, that's only going to work for the aryan ubermench who pick up your book.

Assuming you're printing one-color (ie, black, including greyscale), be sure your artists know this.  That doesn't necessarily mean that you can only take greyscale pieces.  Some color work translates to greys just fine; other pieces with low contrast will turn into grey smudges.  Your artists should be well aware of what will translate and what won't.

As a last point, as you're constructing your wish list, consider how you can use each piece more than once -- perhaps you can keyhole crop elements out of a larger piece to begin a chapter with a little cut-out.  Perhaps you can repurpose some of the art for use on a website.  Perhaps you can, I dunno, make a collage out of three of them and use it as your cover.  Art is a resource like any other; find ways to use it optimally.

A trick I used is, once the artists understands your overall look and genre, let them make any action oriented pieces they feel like.  Then use what's going on in those pictures to base your game rules examples off of.

It serves a sneaky double purpose.  If you get back art that has cool people doing cool stuff, and you can't figure out how that would happen in your game rules...you've got a problem in your rules.  From any given action picture you should be able to figure out 1) how those characters would be stated (however you have stats in your game), 2) what sequence of real people speaking at the table would have led to those characters being in that situation (i.e. GM called for a roll, player spent a hero point to initiate a conflict...whatever), 3) and what section of your rules would be used to resolve.

Boom, you've got an example of a game sequence from start to finish that exactly mirrors what's going on in the accompanying image.  Exactly like matching the picture to the text, but a heluva lot easier on the artist.  Heck, if you're lucky (and your art needs are generic enough) you can possibly get some already existing art pieces the artist has lying around...could be much cheaper to boot.