The book is a compilation on the various Robot Damashii figures released in the past. There is also interview with mechanical designer Ebikawa Kanetaka.
Also, the book will come with the purchase ticket to get Robot Damashii [Side MS] Gundam Astraea.
"Robot Damashii Side Book" will be released tomorrow (April 24th) at 2,100 Yen (inclusive of tax). Pre-order for Robot Damashii [Side MS] Gundam Astraea will end on June 10th. Mid November release. The price is 3,150 Yen (another 800 Yen for delivery and handling required).
Images are from Dengeki Hobby Web.