I vaguely remember when younger a spotter I knew having a book listing all airlines and all aircrafts within the airline at time of print. Is this data available now in book or online format? I'm looking to get back into spotting.
You can find much of this sort of information at http://www.airfleets.net
For example, searching for British Airways lists their fleet, and clicking the number of active for a model (eg A318) will list the tail number and other interesting information. From there you can click a specific airplane for all the information along with a picture.
You may wish to try PlaneLogger, which is an online site for logging your aircraft sightings. It has a large database similar to AirFleets, but allows you to mark off the aircraft as you seen them.