I have been known to be a book-addict since I was a kid, and I don't change that addiction even when I moved to Taipei. In Indonesia, I used to go to bookstore once every two weeks (sometimes every week), to buy any kind of books (graphic novels, magazines, novels).
Moving to Taipei, I don't get to go to bookstore that frequently. First reason: I cannot read Chinese character, meaning my option is only English books. Second reason: English books are not that cheap. Not really expensive, but I need to think before buying them.
There are several specific bookstore I love to visit in Taipei, they are Eslite, Page One, and Mollie Used Book Store.
Eslite
NTU branch: next to NTU main gate (Xinsheng South Rd). Opening hours:  10:00 AM – 11:00 PM (Fri-Sat) ; 10:00 AM – 00:00 (Sun-Thu) Xinyi branch: 11 SongGao Rd, Taipei Opening hours:  10:00 AM – 02:00 AM (Fri-Sat) ; 10:00 AM – 00:00 (Sun-Thu)
Eslite is popular as the largest bookstore chain in Taiwan. It also has a very big and 24-hour store at Dunhua area. I seldom go there because it is far, so I often visit its store in Gongguan area. It is not very big, but provides a quite large collection of English collection. It also has a spacey basement of which many people sitting in the aisle reading promotional books, and it is very nice because it is common in Taiwan, the staff will not hush them for doing so.
(Eslite Xinyi store, personal collection)
I really love visiting Eslite Xinyi store because it has a neat and tidy room interior.  The store occupies 3 floors of its total 8 stories of Eslite Building. There is a huge number of book collections and friendly spaces (including chairs) for short reading, so I can spend a whole day there (reading promotional books). Second floor is for newly arrived books, and Eslite often offers discount for these books. We can check the collection first on its website. English books are also located in the second floor, categorized based on genre and the author. On the other side of the floor, there are collections of magazines, both local and international, like Marie Claire, Cosmopolitan, and Vogue (not just English edition, but French as well!).
More and more books are displayed in the third floor. Mainly in Chinese, but there are several travelling books written in English. I was so jealous looking at some kids reading graphic novels when I visited the store. Not just fiction books, Eslite also houses science books, and some Japanese books in the fourth floor.
Page One
Taipei 101 branch: 44 Shi-Fu Rd 110, Taipei Opening hours: 10.00 AM – 10.00 PM
Page One is located in Taipei 101 mall, in the fourth floor (in front of "city walk"). Last time I went there (a couple weeks a go), the store was in some kind of restoration (or sale, or moving out thing, I don't know), but when I need to buy popular novel, I prefer going to this store to Eslite. More English books collection (because it is an originally Singaporean bookstore chain).
Although it is not as big as Eslite Xinyi, Page One Taipei 101 has these hideous spaces, because we will not really see them until we browse some books to the backside of the store. The front side, as usual, is a spot for new arrivals and bestsellers. Walking through the aisles, we can find many English books and some Chinese books. They are classified neatly in shelves, mostly based on authors, some based on topics. Go around the back, and there is a small way to enter the hideous corners, mainly children books and stationary.
Another thing I like from this bookstore is, Page One always offers discount. It varies from 10% to 40% off, though we must be careful, not every book will end up cheaper than other bookstore. But yes, some are definitely cheaper than others. Some popular English novels, though vary in prices, normally offered at NT$282 (around US$8.5) –I often buy English novels at that price– here in Page One, you will get 10% or 20% off. That’s much, just like when you buy books at TogaMas Bookstore in Indonesia (my favourite bookstore, you get discount, you get free plastic-covering service). Surely, no bookstores in Taipei offer such service (as far as I know).
Mollie Used Bookstore
Gongguan branch: Roosevelt Road Sec 4, No. 13, Lane 24, basement (B1) Opening hours: 10. 00 AM - 10.00 PM
Because I needed to have some textbooks for my study, I asked my labmates where I can find those books in a moderate price. Copying textbook is illegal in Taiwan, so we have to buy the original copy (in English), which costs about TWD 1,200 per book (IDR 340,000). He suggested me to look for second-hand books in this store: Mollie. There are two Mollies in Gongguan area; one is near a Vietnamese restaurant, the other hides in next section of the road; they both sell second hand books and music albums. The first one sells lecture books, some magazines, and CDs/VCDs/DVDs. The lecture books are in a very good condition and usually priced at 1/3-1/2 of new book price. There are a lot of lecture books collections in Mollie, from management to engineering. For purchase of more than NTD 500, buyer will be given a member card and 15% discount for every purchase after.
Another Mollie is a bit hard to find, it is near Discovery restaurant (take the aisle through Roosevelt Rd. Sec 4, and turn right after two other aisles). This bookstore mainly sells fiction books, like novels and comic books. The store has a lot of books in English, like some look-very-brand-new Harry Potter collection. The price is roughly the same, 50-90% cheaper than new books. Dan Brown’s novels, for example, are sold at only TWD 100-150 (IDR 27,000 - 43,500) each.
Let's go buy some books! -Citra