The Clip+ is probably your best and simplest bet at a reasonable price. It satisfies your three main requirements:
1. loads protected WMA audio files. Note that iPods can now load OverDrive audiobooks via a conversion process, but you'll experience a long loading time because of that conversion, compared to loading into the Clip+.
2. has a folder view (under Music).
3. resumes all files stored in folders Audiobooks and Podcasts.
N.B. in order to meet requirement 3 above, you can't just click through when loading the Clip+ via the OverDrive app. Instead, you must force the audiobook to load under folder \Audiobooks instead of the default loading folder \Music (where it would be playable but you wouldn't get the resume function). You get a chance to change the loading location during the OverDrive "Transfer" sequence, by clicking on "Advanced Options..." when it appears, and editing the root of the path from "\Music" to "\Audiobooks".
I hear Cowons are good, but they're pricey (at least in the U.S.). Many Sonys (those that advertise that they support podcasts) can also load and play audiobooks and meet your three requirements. But as with the Clip+, you need to specially load them, in this case under \Podcasts and exactly inside a level 2 folder, like this: "\Podcasts\[someOtherFolder]\audiobookfile01.wma". This is more restrictive than the Clip+ loading, which can see files at any hierarchical folder level under \Audiobooks.
The Clip+ has a micro SD slot while the Sonys don't. The Clip+ and Sony can play back faster than normal, but the Clip+ gives the "chipmunk" sound, while the Sony does pitch correction very nicely (no chipmunk sound). The Sonys I'm thinking of, like the NWZ-E350-series (mini-USB connector) and -E460-series (proprietary Sony connector) can play video on their 2" screens, though the E350-series doesn't play as many video formats as the E460-series player (it took me a while to find a conversion program that worked for an E354).
Microsoft Zunes also meet your requirements, but you're now complicating your life with an ecosphere (Zune Software is required for most things). And Microsoft no longer makes Zunes. I wouldn't recommend going that way at this point for any but an enthusiast, though I love my Zune.
Creative Labs players don't need any special OverDrive loading steps like the Clip+ and the Sonys, but when you want to pause and it eventually shuts down, you don't get resume. Instead, you have to shut it off while it is playing, and then you'll get resume. These players *do* offer manual bookmarks unlike the Clip+ and the Sonys, but have an unreliable resume (which only applies to the last-listened-to content).
If you want to go Apple, and you don't mind waiting the conversion time when loading, you need to ensure that all the library audiobooks are able to be converted. When I search my library for all audiobooks, it lists 6951. Then when I search for all iPod-compatible audiobooks, it lists 1000. But it has listed 1000 for about a year, so I think that may be a mistake. But I'd ask your library first, to ensure you're not limiting yourself to a subset of your library books.