My friend and I had a conversation the other day about manga in the U.S. and in Japan. I know that in the U.S. manga is sold in the form of a book. My friend said that One Piece and some others are printed in a weekly magazine. I just wanted to know if manga in Japan is printed in the style of a book or like American comic books or perhaps they come in both. I tried to google it but I couldn't find a very good answer.
The information that answers your question is on the Wikipedia entry for Manga.
They describe the general process about how manga is distributed starting at the "Magazine" level:
Eshinbun Nipponchi is credited as the first manga magazine ever made. Manga magazines usually have many series running concurrently with approximately 20–40 pages allocated to each series per issue. Other magazines such as the anime fandom magazine Newtype featured single chapters within their monthly periodicals. Other magazines like Nakayoshi feature many stories written by many different artists; these magazines, or "anthology magazines", as they are also known (colloquially "phone books"), are usually printed on low-quality newsprint and can be anywhere from 200 to more than 850 pages thick. Manga magazines also contain one-shot comics and various four-panel yonkoma (equivalent to comic strips). Manga series can run for many years if they are successful. Manga artists sometimes start out with a few "one-shot" manga projects just to try to get their name out. If these are successful and receive good reviews, they are continued. Magazines often have a short life.
Then in the next paragraph talks about the collected volumes:
After a series has run for a while, publishers often collect the episodes together and print them in dedicated book-sized volumes, called tankōbon. These can be hardcover, or more usually softcover books, and are the equivalent of U.S. trade paperbacks or graphic novels. These volumes often use higher-quality paper, and are useful to those who want to "catch up" with a series so they can follow it in the magazines or if they find the cost of the weeklies or monthlies to be prohibitive. Recently, "deluxe" versions have also been printed as readers have gotten older and the need for something special grew. Old manga have also been reprinted using somewhat lesser quality paper and sold for 100 yen (about $1 U.S. dollar) each to compete with the used book market.
There is also a link to a list of "Manga mangazines" here.
One Piece (Japanese: ワンピース Hepburn: Wan Pīsu?) is a Japanese manga series written and illustrated by Eiichiro Oda. It has been serialized in Shueisha's Weekly Shōnen Jump magazine since July 19, 1997, with the chapters collected into seventy-eight tankōbon volumes to date. One Piece follows the adventures of Monkey D. Luffy, a funny young man whose body gained the properties of rubber after unintentionally eating a Devil Fruit. With his diverse crew of pirates, named the Straw Hat Pirates, Luffy explores the ocean in search of the world's ultimate treasure known as "One Piece" in order to become the next Pirate King.
Shounen Jump is described as
Weekly Shōnen Jump (週刊少年ジャンプ Shūkan Shōnen Janpu?) is a weekly shōnen manga anthology published in Japan by Shueisha under the Jump line of magazines. It is the best-selling manga magazine, as well as one of the longest-running; the first issue was released with a cover date of July 2, 1968. The manga series within the magazine target young male readers and tend to consist of a large number of action scenes and a fair amount of comedy. The chapters of series that run in Weekly Shōnen Jump are collected and published in tankōbon volumes under the "Jump Comics" imprint every two to three months.
Clearly stating that it is a manga magazine, and outlines that series are collected and published in tankoubon volumes.
The "One Piece" wiki entry also states (at the time of this answer), that there are
seventy-eight tankōbon volumes to date.
And the wikipedia entry for Tankoubon describes it as:
A tankōbon (単行本?, "independent/standalone book") is the Japanese term for a book that is complete in itself and is not part of a series or corpus (similar to a monograph), though in modern Japan it is most often used in reference to individual volumes of a single manga, as opposed to magazines (雑誌 zasshi?), which feature multiple series. It can be used for a novel, a nonfiction work, an economics textbook, a book of beauty tips, a book presenting a coherent set of photographs, an exhibition catalogue that samples earlier books, and so forth in a hardcover format. It is more specific than plain hon, which encompasses such books but also one or more issues of a periodical, one or more volumes (or the whole set) of an encyclopedia, etc.
This information is slightly more detailed than required by your question, and a simple search would have hit the Manga entry in Wikipedia.