Here's something I learned just the other day, and it may affect what kind of player you're looking for. My little 1gig flash drives are Creative Zen Nano Plus (Plus means it has an FM radio, not that I use it). Anyway, Lately I have been listening to books on it. It plays the tracks aphabetically in one folder. So my 9 CD book is ripped into tracks that look like 1a, 1b, 1c ... through 9a, 9b, 9c.
Now the "other" kind of players which we tend to think of as ones with a hard drive... those do NOT play songs from in a folder, nor do they play songs in an alphabetical order. They keep track of Albums, and the songs or tracks are played in the order they were on the CD/Album from which it was ripped. This is great for music... not so good for books. For example my 9CD book above will play in this order on this type of player 1a, 2a, 3a ... so I hear the first track on each of 9 CDs before it plays track two on the first CD. Not good for books. I said we think of these as having a Hard Drive, but in fact my friends recent Creative Zen V Plus is 2g and its NOT a hard drive but rather some kind of flash memory and it handles the files this way.
Bottom line is this. These two players play a different way. If books are you're thing, buy the first kind and be wary of the second kind, if music is your thing, then probably the second kind is better for you.
As for brand, I have 3 Creative Zen Nano Plus(s) for easy use and small size. They use a standard alkaline/sp AAA battery. It truly does last about 15 hours. My wife and I also have 2 Creative Zen Jukebox Xtra (now out of production because of age). One is 30 gig, the other 40 gig. They have a rechargable Li-Poly battery that is easily replaceable. They last a good 12-15 hours of play. Ours are close to three years old and still on the original battery. Two friends have the same unit and are on their second battery, but I think they burned 'em up by leaving them on the charger all the time. Replacement batteries run $25-29 from on the Web, or $49 directly from Creative.
Good luck in your search.