Based on the title "The Rainbow Kueh Book", it is highly likely you are referring to the popular cookbook by Singaporean author Angie Tang (often associated with the EGW Centre for Seniors). The book is a tribute to the art of making Kueh Lapis (Rainbow Layer Cake) and other traditional Nonya kuehs.
Here is a helpful overview of the book, what makes it unique, and why it is a valuable resource for bakers. the rainbow kueh book
While the "Rainbow" Lapis is the star, the book typically covers a variety of traditional steamed and baked kuehs popular in Southeast Asia: Based on the title "The Rainbow Kueh Book"
If you are using this book to bake, here are three helpful tips to ensure success: Kueh Lapis Legit: The signature baked layer cake
Due to high demand (especially during the Lunar New Year and Hari Raya seasons), The Rainbow Kueh Book often sells out in physical stores like Kinokuniya and Times Bookstores. It is currently available in hardcover (collector's edition with a gold foil cover) and spiral-bound (preferred for kitchen use because it lies flat).
You can order the official copy through major online retailers or the publisher's website. Be wary of low-quality PDF scans circulating on forums—they lack the color accuracy needed to judge the "doneness" of your batters.