The kinds of things I'm interested in are things I can work on alone, such as stretches, tenkans, (seiza, yes, it still hurts like hell over 30 seconds!) the different names of the techniques, but I think most importantly, ukemi. As an 'oldie' they are the hardest part of it all, because my dojo doesn't have a seperate beginners class, and we do techniques for 6th and 5th kyu which require some 'head over heels' action, and I kind of go into a cold sweat just thinking about them..
. Also, any book that has good picture/explanations of detailed hand movements would be good because I wear glasses, and taking them off before training means I sometimes don't get a good look at the specific positions of hands during demonstrations.
Edit: Oops, I forgot, maybe I should mention that I train in the Aikikai style!
For Aikikai style Aikido I would get either " Aikido and The Dynamic Sphere or "The Structure of Aikido" by Gaku Homma.
" The Structure of Aikido" focuses on ukemi (falling) and basic body movements. Over 1600 photos for more than sixty techniques shown using the ken and open hand.