I noticed no one has reviewed this yet, so I thought I might as well do it myself. I have read all the volumes released in the USA (5 of 7).
These stories aren’t in the Manga and weren’t written by Arakawa (creator of FMA manga). However, Arakawa supervised the creation of the novels and helped create the stories. So, I would consider the information in these books to be an actual part of the FMA world and not just "filler." One of the stories even was animated in the original Fullmetal Alchemist anime.
These are a series of light novels. Each novel is about
220 pages or so and are an easy read. Don't expect some literary masterpiece that you can analyze; these books are to be exclusively read for fun. Also, the series is not necessarily continuous in the sense that you can pick up the 4th novel without reading any of the other ones. Each novel is independent and has its own story. Also, you don’t really need to have watched or read FMA to understand these books. The books give no spoilers and each book spends a few pages describing Ed and Al, their bodies, their journey, etc. However, I don’t think it introduces the characters enough to make them as loveable as the manga and anime do. So, it’s possible to read these without knowing about FMA, but I would mainly recommend it to FMA fans.
There are 7 books in total in the series. So far, 5 have been translated into English. The fifth book was translated in 2007, so I'm unsure whether or not they'll ever be fully translated. It's shame really because these books are not bad at all. Some of the books are one full story (full 200+ pages), others have a long story (180 pages or so) and a short story (40 pages or so), and others have two stories (100ish pages each).
The plot for these stories isn’t very complex or mind-bending. Usually you can see what’s going to happen from the start. Usually it goes like this, Edward and Al are out looking for a philosopher’s stone, notice something seemingly good, learn the true bad side of the situation/person/thing, and save the day. Some of the stories do, however, reveal some information about FMA that wasn’t in the manga or anime. For example, in book 4, we learn about one of Ed’s best childhood friends named Pitt.
As for extras, every book has a full color image as the first page, and a few black and white sketches throughout the book. There is always a humorous prologue from the author (Makato Inoue) and sometimes a short, 1 page, funny manga strip.
The US translation occasionally has some grammatical errors and awkward sentences. The most grammatical errors were found in the first book and much fewer can be found in later books. The books read in a very informal manner as if one were talking. I highly recommend this if you can’t get enough FMA.
For information purposes, here is the ISBNS for the books:
Book 1: The Land of Sand ISBN 978-1-4215-0155-0
Book 2: The Abducted Alchemist ISBN 978-1-4215-0222-9
Book 3: The Valley of White Petals ISBN 978-1-4215-0402-5
Book 4: Under the Faraway Sky ISBN 978-1-4215-1397-3
Book 5: The Ties that Bind ISBN 978-1-4215-1431-4
Book 6 (Japanese): A New Beginning ISBN 978-4-7575-1984-8
Book 7 (Japanese): Daughter of the Dusk ISBN 9784757528666