I used the word "merely".Hard Copies of books are more easily skimmed, highlighted, revised and then retained as well.You can write side-notes containing the facts and figures and half of the work of memorization gets completed.You can read book any where, where there is quiet and calm niche, take it on your roof, under the tree, in the sun, sleep it making your pillow, and then wake up, start reading again...No stress on eyes as you have to study continuously for 12-16 hours. Books have usually more authentic material if the versions are latest. It has material at one place in compiled form, chapter by chapter, clearing your concepts and synchronizing the thought.
Digital copies of many local books are not available e.g an excellent book for this subject written by Maleeha Lodhi...Local books have more comprehensive, to-the point material sometimes (though can be fallacious and un-authentic).

At the end of the days, you will find it more helpful to have your material at one place, authored by one person, with one style...You know every author has it's own style, and you get baffled sometimes "merely" with different styles during CSS studies...

Even material present on internet is sometimes outdated...Even two authentic sites will have different facts and figures. Even sometimes internet get's hiccoughs, and refuses to give you the required material.

So, material from internet has it's own importance, but you can't deny that 1,2 books with relevant material, have a relieving effect on your nerves while preparation. Every one can't afford the tabs and such latest technology. I know a CSP officer who only relied on the scrap material( "Raddi"), second,third hand books and then someone gave him the membership of library...It's 2015, but poverty and poor people still exist in Pakistan.
D.ps seldom reflect our personalities, I guess.