This post was written with MOSS 2007 in mind. If you need guidance on SharePoint 2013 mobile offerings, go here
This is a complicated topic. You've may have seen the ability to mobile browse lists, libraries and list items by appending an /M to the site url. You can do that with Publishing Sites too, but the /m feature is disabled by default. You can re-enable it by using the following STSADM command;
stsadm -o activatefeature -name MobilityRedirect -url http://yoursiteurl/
Once that runs successfully you can append /m to your site url to see Lists in a mobile view (but not the Pages list). [btw Even without activation of the MobilityRedirect feature you can already see the mobile view of your WCM site with the lengthier url http://yourServer/yourSite/_Layouts/mobile/default.aspx].
If you want to display your WCM pages in a mobile device that's where the complications start. The WCM pages aren't viewed in quite the same way. There is a quick and dirty way, and a proper way to display WCM pages to mobile devices. The dirty way is mobile views, and the proper way is MOSS Variations.
Quick and dirty solution is to create a mobile view for the pages folder you want to display.
Site Actions -> Site Settings -> Modify Pages Library Settings
At the bottom of that page there is Views -> Create View -> Standard View
Under columns, select and deselect items so that it will be showing what you want.
Create a public view. In the mobile section at the bottom of this page check "Make this a mobile view" and "Make this the default mobile view" if you so wish.
OK the creation of that view. The view is displayed in a normal sharepoint page.
Click the dropdown -> View -> Modify View
In the Mobile section at the bottom you'll find the web address for this mobile view. If you paste that address into the browser, you'll see the view as a mobile device would see it. You can click the "Display" link on pages and see the content. It isn't pretty though. (But it's easy).
The nice way involves Site Variations.
There's likely a range of mobile devices you want to provide your WC for. You'll have to use the Variations feature to create site variations whose page layouts and styling support the classes of mobile device you want your content to be browsable on. One variation for Windows PDAs perhaps, another for Symbian mobile phones.
From my own experience on a major mobile WCM project, it's a major investment to get rendering right for a range of mobile devices, depending on what you want to push out. (http://mobile.transportdirect.info/)
One of the higher profile Sharepoint mobile web applications is GLU.COM which was blogged about here; http://blogs.msdn.com/archive/2007/06/14/moss-has-got-game-glu-mobile-s-website-www-glu-com-how-we-did-it-part-1-of-3.aspx
You might also want to check out the ASP.NET mobile device forums for more of the gotchas. http://forums.asp.net/48.aspx
So, there's no easy answer to mobile access I'm afraid, and no single comprehensive pool of documentation that I'm aware of. MSDN comes closest at http://msdn2.microsoft.com/en-us/library/ms462572.aspx?PHPSESSID=tn8k5p1s508cop8gr43e1f34d2
As for books, the only places I have seen any decent guidance in print are Andrew Connells WCM Development book, and Nirav Mehta: Mobile Web Development. Andrews commentary on Mobile Web is Sharepoint specific, whereas Nirav covers in detail the real gotchas in mobile web development.
As for the what devices can be used. Web enabled basically. (Windows PDAs might be a better choice, as you may want your clients to view MS Office file types, and the file readers will be more available on such devices, but this isn't essential for viewing the WCM pages). You need to think about authentication methods for access. Is this site going to be anonymous access? If not, forms authentication is likely to be the way to go from past experience, depending exactly on the type of mobile client. (In Japan, NTT DoCoMo phones have unique registered numbers that allow an additional form of authentication, but that is Japan only. So perhaps it's overkill me mentioning that!) Let's face it we're talking forms authentication!
Update 13-01-09 I recently noticed this documentation on the supported browsers for MOSS mobile pages;