What should I read in English?
If you wish to maintain, or improve, your high-school English-reading skills,
you must keep reading. But what should you read?
You can read whatever you like! You do not have to read difficult English
all the time. In fact, if you find something simple that you enjoy reading,
you are more likely to a) read a lot and b) continue reading every day.
But here are some suggestions:
1. Books for practicing reading.
On the third-floor east-facing side of the library (3F東), there is a section
with books for learners of English (語学学習コーナー). The books are simplified
versions of famous English books. They contain easier vocabulary and have
been made shorter. There are various grades, so choose one that is around
your level, or below. Before you read, you could look at a Japanese translation
of the book. Afterwards, you could try reading the original, unsimplified,
English book.
Reading these books will help you to gain experience in reading in English
(without translating into Japanese) and ease the transition from high-school
to university English. 
2. Books that have been translated into Japanese.
If you want to try reading more difficult books, it might be easier if
you read a translation first. You might be surprised by the large number
of English books that have been translated into Japanese. In many cases,
the library will have both versions. For example, you could read Michael
Sander's Justice in this way.
3. Newspapers and magazines
The Hiyoshi library has recent editions of both Japanese and English newspapers
on the first floor. The Japan Times is a good newspaper to start with as it contains a lot of news that you
should already be familiar with. On some days there are special sections
for English learners. 
If you want to read an English-language newspaper that is published overseas,
such as the New York Times, it might be a good idea to start with articles about news that has also
been covered in Japanese newspapers or in the Japanese TV news. If you
are already familiar with the basic facts of the story, it will be easier
to guess vocabulary etc. (Of course, it is also interesting to compare
how the newspapers of different countries report the same story...)
News magazines provide longer articles on topical stories. Newsweek is probably the easiest to read, then Time, then the Economist. Of course, there are many special-interest magazines, but they will not
necessarily be in the library.
Note that the Mita library has a much wider selection of English-language
newspapers and magazines available. 
4. Reading online.
English is the main language of the internet! Whatever your interests,
you will probably be able to find English-language websites about them.
Of course, some sites are more reliable than others. 
All the main newspapers and magazines have websites, and you will be able
to access many of these from your home. Again, it is a good idea to read
stories that you already know about. You could also see how different newspapers
report the same story. For example, you could compare how the Japan Times, the People's Daily and the New York Times report the dispute about Senkaku/Daioyu. The BBC News site gives access to news about countries that are often ignored by Japanese and American newspapers, particularly those in the developing world, such as countries in Africa. 
You can find a list of the websites of "the Top Ten" English-language magazines and links to the "Top Ten" magazines in different categories (such as men's or women's magazines) here.
There are many sites for English learners. For example, the BBC (the British
equivalent of NHK) has a special Learning English site; so does the British Council, a government agency similar to Japan's 国際交流基金.
If there is any more information you would like on this page, please tell
me!