The first/original Generations book was first published in 2004, and covers all of the Hasbro and Takara Gen1 and Gen2 toys, plus some unreleased items and concept drawings.
It is one of those "must have" books for Transformers Gen1 toy collectors or fans, and even though it is mostly in Japanese, there hasn't been a better book published in English... so is highly sought after.
Since 2009, a Japanese publishing company has been producing annual Generations books, of a similar style, but covering toys from the current year (or just from a few months, as some years had so many toys, there have been 2 or 3 volumes produced for that year). And some volumes will include pre-2008 toylines, so that eventually, all Transformers toylines will be included in the Generations reference book series.
(I think the Beast Era is the only one left now, and since this year is a Beast Wars anniversary, I'd be betting that the 2016 Generations book will include the Beast Era toys.)