Hi everybody I would like to offer you my solution to this issue though I must tell you it is a poor one but effective.
As I faced the same problem as you guys when I upgrade to Maverick and could not load books in Ipad 1 I did the following:
1st. Downgrade to Itunes 10.7, there are some instructions on how to do it on the web also videos in youtube worth to check.
2nd. I had to formatIpad 1 so make a back up previously.
3rd. Once the Itunes 10.7 is installed and your ipad 1 is "brandnew", you can use Calibre for uploading your books to Itunes and also to transfer these books to Ipad 1, is in that moment when you will encounter our wellloved error 42408. in order to solve it please go to step 4th.
4th. Seems to be that there is a solution a bit more complex that te one I am going to offer you also more useful so look for it and take your time, I didn´t do it cuz yesterday I spent 15 hours dealing with Itunes and I was feed up, so IO took the short path. I resumed that the quickest solution was to upgrade again to Itunes 11 which doesn´t solve the problem at all but it is also true that Itunes 11 I am able to sincronyze Ibooks to ipad 1 (having previously added 1400 books form Itunes 10.7) so now I can read again in Ipad 1
I Know that it is a very crappy solution, but at least is not as crappy as the one offered by apple and all its enginners.......Hope t is a momentary solution to all of you and that in the incoming future somebody in apple decide to stop punishing our ***** for the fact of not having its latest hardware.
Saludos a todos!