One has to do some physical work as part of the Karma Yoga, also some personal work. Where is the time then to read for one’s mental development except at the cost of the time spent in meditation?
If the power to meditate long is there, a sadhak will naturally do it and care little for reading, – unless he has reached the stage when everything is part of the Yogic consciousness because that is permanent. Sadhana is the aim of a sadhak, not mental development. But if he has spare time, those who have the mental turn will naturally spend it in reading or study of some kind.
For one who wants to practise sadhana, sadhana must come first, – reading and mental development can only be subordinate things.