Had a question via PM about maps for ebooks. I can only relate my own experience & thoughts on the matter, so here it is.
SCOPE -- There are two initial approaches: designing for some specific sort of reader, or designing for "all" readers. If you target a specific reader, such as the Kindle 3 or iBooks on the iPad, then your design parameters depend entirely on that reader. But I think most projects are going to want to be viewable on a range of readers, and so we must pitch more or less toward the lowest common denominator.
DEVICES -- First, let me throw out the iPhone (yes, it's an ebook reader! One of my friends read an entire novel on his ... well, okay, Android phone, but you get the point), because the phone screens are small, maybe around 300 pixels or so, and if you design for that low a resolution you've kinda screwed yourself, and who reads books on phones anyway except for my crazy friend? So let's just go with the ereader hardware and software. The hardware includes Kindle, Nook, and such e-ink clones; the software includes iBooks on the iPad, various other proggies and apps such as Kindle Reader, Adobe Digital Editions (ADE), Nook for PC (a piece of stinking shyte, imho), etc.
FORMATS -- Next, let me throw out PDF entirely, because that's not "ebook." I mean here only Epub, Mobi (the "old" Amazon format: azw, prc), and KF8 (the "new" Amazon format). Not interested in other kinds of ebooks.
LCD -- The lowest common denominator for those formats on those devices is grayscale at about 500x666 pixels, portrait orientation. Why grayscale? because there's no color e-ink yet (maybe a year out). Why 500x666? because though most e-ink readers are 600x800, usually they impose margins around the edge, and their downsampling of images is nothing like Photoshop or even Firefox/Chrome ... you get nasty jaggedly downsamples, looking like cheap GIF shrinks. So, to avoid getting downsampled, use 500x666, which is proportionally the same aspect ratio as 600x800. And why portrait orientation? because that's the default orientation, and people are brainless and don't even know that you can change to landscape ... and nobody reads in landscape anyway! so you don't want to make people turn their ereaders sideways all the time ... and it's really difficult to turn a laptop sideways, or a desktop-PC monitor, for those who are reading on software readers instead of e-ink devices. Thus, the lowest common denominator: grayscale 500x666 portrait.
TYPE -- Next up is the problem of fonts. You can only go so small. E-ink devices are usually higher pixel density than computer monitors: your computer screen (where you're designing your map!) is probably not much greater than 72dpi, but (roughly) the iPad is 130dpi and the Nook & Kindle are 170dpi -- much smaller pixels. What looks fine on your compy will be a tiny squinty "omhygodimgettin'old" mess. Because every font is different in its point size and clarity, the only way to find your smallest readable type size is to experiment: make an image with various sozes of your font(s), put that image in an ebook, and look at it on your Kindle or Nook. It will loook huge on your computer screen, and you'll be tempted to shrink it down; don't do it! or, go ahead and do it, and run your test again. I'd love to tell you to use only san-serif fonts, but of course we can't always use the fonts we want. Remember this, though: the fonts that come on the Kindle, and especially its default font, Caecilia, are chosen because they are legible on e-ink screens: not a lot of stroke-width variation, no fine pointy serifs, etc.
TROUBLE -- Now, I don't know about you, but I try my damnedest to follow the "north is up" rule at all times. Some maps will want to be landscape with north at the top. What will you do? I don't know what you'll do ... shrink the map? cut it into left & right pieces? offer a reduced "overview" map and then one or more zoom-in detail maps? Every project will have its own unique demands, so I can offer no help on that point.
GRAYS -- Lastly is the matter of levels of gray. Again, every reader is different, but in general we only have a handful of levels of gray to play with. E-ink doesn't display nearly as cleanly as LED pixels Refreshes are a problem, with ghosting. I suggest using as few levels of gray as you can get away with. Small gradations such as drop-shadows will probably be fine; the larger you get, the more problems you'll have. If you want to indicate ocean depth or terrain elevations with smooth gradients instead of boundaried steps (e.g. one shade every 10 meters), it might look like crap on an e-ink device. There is no substitute for testing.
In closing, let me say this: ebook technology is in its infancy, and standards (as usual in the digital world) are sketchy at best. Test on the major devices -- Kindle, Nook, iBooks -- and as many others as you & your friends & netbuds can get your hands on. When it comes time to compromise (and it will!), compromise in favor of the major devices. It's better to make a map that looks good on a Kindle and really junky on a Cybook, than to make the map look mediocre on both. I mean really, who owns a Cybook?