I've been writing essays preparing for an IELTS exam, and writing this particular paragraph i'm not 100% sure whether this is the right usage of the expression "in which".
Whereas in the past books were the main source of knowledge, the arrival of the internet has drastically altered the relevance of books, because of the easiness in which such information can be accessed.
Is this the proper usage of the expression?
You could use by which as @mplungian suggets but it would be more usual use with which giving "...the ease (not easiness) with which information can be accessed".
It is difficult to state a rule for this but the nearest I can come to one us that by which is usually associated with the method used to do something, with which is usually associated with the manner that something is done and in which, on which and so on are associated with where something can be found. So you have "the method by which he searched", "the speed with which he ran", "the book in which I found", "the table on which it stood". There are a few odd uses like "the car by which he stood" but, in that case, the preposition by is being used in the same way as in. It's the same as "the car in which he sat".
Having said that there is also "the way in which I searched" which looks as though it should be the same as "the method by which he searched" or "the speed with which he ran" but, actually, 'way' is a not really a noun defining a process like 'method' and is not a descriptor of an action like 'speed'. As I said, stating a rule is difficult, if not impossible.
by whichis better I would think - but such information does not work here either because that does not match main source of knowledge. I would consider Whereas in the past, books were the main source of knowledge, the arrival of the internet has drastically altered their relevance, due to the ease of accessing information online.