Beginning slide question - tunings, basics and fun!

First off - just one big thank you to all of you who take the trouble to reply to guitarnoise questions!

I've now been playing coming up two years (mid-life born again guitarist, with more money than sense). About a year ago I bought a cheap old Framus blueridge guitar with a D'armond pick-up with a view to playing acoustic (I already had an electric, which I use for lessons). This guitar when it arrived had a thin neck with wide frets, high action and really heavy strings and after a week, I gave up with it and went back to playing the electric.

Last month I bought three books on slide playing, including an Elmore James songbook, and a bottleneck. I picked the guitar up and following the books instructions tuned it to Open D tuning (D chord on open strings). The following few days I could hardly put it down. I've obviously made some improvement over the year but the fun from playing basic 12 bars on this and the'Dust my broom' lick is immense.

Now my basic questions - pretty much all the songs in the Elmore James are Open D tuning, none of the other books have a song in this tuning, now I'm presuming Elmore didn't have a unique singing voice so why use this tuning? Also a lot of the other songs are in Open E - now I'm presuming all 'licks' and scales for this tuning are identical to Open D but moved just moved a tone?

I've seen 'Drop E tuning' mentioned is this the same as Open E?

Also one song book has songs in 6 different slide tunings, is the main point of this to make certain chord/lick sequences easier as well as adding colour?

Finally can anyone recommend any blues tabs for standard tuning on an electric?

Thanks again
Martin