Notes 8 / 8.5 is a radically different animal than 5 or 6. Things developed for 5 and 6 still work under 8 but there are substantial capabilities that are new to 7 / 8 / 8.5 – composite applications and xPages are the newest and unlike much that has gone before. While “Lotus Notes Domino 8 Upgrader’s Guide” from Packt Publishing is not strictly about development, it does have enough to give you a feel. I’ve not seen the usual multi-pound books from Que or McMillan yet (don’t get me wrong, I recently recycled my Notes 5 Unleashed and still have my HTML 4 book at arm’s reach). I expect to see something from them soon.
That said, there is a wealth available free online, if a bit disjointed, on SearchDomino, IBM DeveloperWorks, and several blogs. If you can tolerate a bit of search time trying to find the best explanation for a particular topic, this may do you well until the publishers catch up.