Extensive reading basically means reading a lot. It is the kind of reading you often do in Japanese for pleasure, such as reading a novel or magazine article. (It is, therefore, quite different to a lot of reading you do in university, where you read a short text slowly and carefully in order to try and understand it). Extensive reading can be a great way to improve your overall English proficiency because it provides you with a lot of English 'input'.
To practice extensive reading in English, you need books that you can read quite easily. Luckily, there is a kind of book made for that purpose. They are commonly known as 'graded readers' and your new library has several hundred of them.
Graded readers are books designed for learners of English that can be read without difficultly. They are also shorter than regular books in English and can be read quite quickly. They are published in different levels of difficulty, so you will be able to find books that are just right for your level. Put simply, choose a book that looks interesting for you and which you can read quickly without using a dictionary. I think 80-100 words per minute is a good reading rate.
You can find graded reader books in the new library on the first floor on rows 08, 26 and 27. The best books are the Oxford Bookworms but the Macmillan books are also very good. The Penguin books have nice covers and look great but they can be a bit harder to read.
I also have a quite a lot of graded readers in my office, which you are welcome to borrow anytime.
Finally, if you choose a book that is too hard or a bit boring, stop reading, take the book back and choose a better one. Extensive reading should be enjoyable.