Very unusual to go from NG to Classic but shouldn't be a problem.
Lots of minor system differences but it is basically the same aircraft. The biggest differences are electrics and flight instruments, I would concentrate your studies on those.
As far as the flying, you will have to get used to identing navaids again and possibly position checks depending upon the vintage of aircraft and FMC update you have. Also the Classics use a fixed flap speed schedule rather than actual weight speeds on the speedtape. On the up side it handles much nicer than the NG, give me a Classic in a howling crosswind any day!
As for a book, may I humbly suggest
this one.
S&L