What does the word “practical ” mean in “Old Possum's book of practical cats”？
Solution 1:
The title is a spoof of title-types common in 18th- and 19th-century popular literature, as if to suggest, very much tongue in cheek, what genre of work the collection should be read as.
Old X's ..., for instance, suggests compendia of popular and esoteric wisdom:
And Book of Practical X is the standard 19th-century title for how-to books:
Note that possum is Latin for I can.
It is quite possible that if Eliot had written this two generations later he would have called it Cat-lore For Dummies.