00. Digital version

You can buy a digital version of the main Japanese/English edition digitally through Amazon's Kindle store (Japanese Store) or Apple Books store (Japanse Store). The book will look quite good on any bigger PC screen or tablet!

0. If you are in Japan

If you are in Japan you can buy the “Tokyo at Night” book in most bigger book stores around the country and online on Amazon JP. If in Tokyo, it's best to go to the main Shinjuku Kinokuniya store.

1. Buy directly from the publisher in Japan

This method allows you to buy the book directly from the publisher MdN, from Japan using Amazon JP. Buying from Japan requires setting up a separate account on the Japanese version of the service.
Order On Amazon JP
If you don't know how to order on Amazon JP from outside of Japan have a look at this TUTORIAL.

2. Buy from retailers selling Japanese goods

Simplest (but sometimes a bit more expensive) way is to search using your local Amazon or eBay websites to look for retailers and shops re-selling the book.
For example, here is the book on Amazon US: Order from reseller on Amazon US

You can also buy the book using some websites that specialize in selling Japanese goods (such as CDs, games, books, etc.) to clients all over the world. If you would like to order the book online but don't want to use Amazon JP this can be a good option. The websites mentioned below are only a few that I know of but there are more. Sometimes the book will be "out of stock" but should be restocked regularly.
Order On CD Japan Order On Otaku.com

3. Buy from Kinokuniya

If you are in USA, Kinokuniya USA online store is selling all my (and Kana's too) major books: Kinokuniya USA store search

You can buy the book in most of bigger book stores in Japan including Kinokuniya stores. Because this major book store chain has physical shops and online stores also outside of Japan you can buy the book in most of them directly. The book is restocked regularly. Below is the list of Kinokuniya stores that should have the book, but before going out please contact the store staff to confirm:

US
• New York main Store
• New Jersey Store
• Chicago Store
• Texas Carrollton Store
• Texas Plano Store
• Texas Austin Store
• Seattle Store
• Beaverton Store
• San Francisco Store
• San Jose Store
• Los Angeles Store
• Santa Monica Store

Australia
• Sydney Store

Taiwan
• Taipei Breeze Store
• Hanshin Arena Store
• Taipei Tienmu Store
• Taichung Chung Kan Store

Singapore
• (Main Store)
• Liang Court Store
• Bugis Junction Store
• Jurong store

Malaysia
• Kuala Lumpur Store

Thailand
• Bangkok Store
• EmQuartier Store

Dubai
• Dubai store

If you know about other, easy ways to buy the book, please let me know!