How much reading is too much?

It depends on what kind of book is in his hand, what kind of content it contains. What is discussed in it?

If a book is really based on philosophical topics or very subtle issues, then reading one book in a week can be excessive because it requires not only reading but also understanding and then digesting it. As Sir Francis Bacon said:

Some books are to be tasted, others to be swallowed and others to be chewed and digested: that is some books are to be read only in parts, others to be read, but not curiously, and some few to be read wholly, and with diligence and attention

On the other hand, some books are so useless & worthless that they should be put aside once they are seen. In such a case, a person must pay attention to another book, in this case, he can read two or more books in a week.

In short, just by looking at the book, it's content and thickness, one can estimate how many books a person should read in a week.

I have used useless & worthless in the following sense.

Every book is not useful for everyone. A precious pearl is worthless to a hungry hen, a grain of wheat is more precious to him than a pearl. The same case is with the book to someone, it will be worthless to someone like a pearl.

The book doesn't need to have no value, but a particular person has no interest in the topic of the book. When he will come to this book, definitely he will put it aside.

Sometimes a book may be written by someone who is not proficient in his knowledge, or his style is difficult to understand. (Especially in underdeveloped countries, where knowledge is not so profound, many books are just compiled like a newspaper, has no value at all)

Some books are very simple, commonly understood and very interesting. One can understand them only by going through it once. Even then, he could read another book in that week. And it won't count as excessive.
In the end, I"ll repeat the first point, I did not mean to despise the book. Rather, it meant that a book was useless to an individual. Otherwise, the book should be respected and honoured.