Isabel and Caroline Bercaw, it's 144 pages published by Quarto-Rock Point and available in ebook and hardback formats.
I am a huge bath geek. In my opinion, there are -very- few problems that aren't seriously ameliorated by a long soak in a tub (for really BAD problems, add red wine (to a glass, not to the bathtub) and a water-resistant ebook reader. Rinse and repeat. If you buy bath bombs from the boutique stores, it leaves a significant dent in the wine budget, so learning to make your own is a win-win situation.
Additionally, a lot of ready made bath bombs have greasy cheap additives and bulk fillers... along with scary artificial colours and other goop.
This book gives you a fun alternative to ready-made. You get to choose the scents and colours (if any) as well as the size and ingredients. The instructions are simple enough, in theory, and would be a fun project with kids if supervised.
There are dozens of recipes, including all sorts of bombs, fizzers, foams, scrubs etc.. along with lots of suggestions for homemade beauty products including lip balm and creams, soaks, scents, masks and toners/spritzes.
The book is exuberant, well photographed and suitable for all ages (though aimed at teen/tween female audiences).
There's a short but handy resource guide (mostly for North American readers) and a useful index to recipes included in the book.
The world desperately NEEDS more tub time in my humble opinion.
PS Pro-tip: you can turn ebook reader pages with the tip of your nose, nullifying the need to dry your hands on a towel or put down your wine glass.
Disclosure: I received an ARC at no cost from the author/publisher for review purposes.