If a man with with long-sighted eye want to
a' text book, what shoðld be the distance between
book and the lens? Give reason

Lots of factors play a role. Firstly if he is presbyopic he won't be able to read without specs at normal reading distances. Many years ago a chap by the name of Donders published a table which shows a linear relationship between age and ability to focus. The ready made reader market relies on his findings to suggest the power you need for reading at different ages. Secondly if he is far sighted (needing correction at distance to see clearly) it will also influence his ability to see close. For example +3.00D distance Rx at an age of 40 years will be very different than the same prescription at age 20 years due to his ability to accommodate. My suggestion is to have comprehensive eye exam to find out what you need at the specific working distance (computer or laptop or book reading all have different reading distances).