Open one of the ideabooks. Immediately under the name there is a row of buttons. Click on the edit button and select the pictures you want to combine. Go to the top of the page and click either copy or move and it will allow you to transfer the pics to an existing book

Thanks for the tip!

I am on my iPhone and don’t see such a command. I want to merge idea books on my iPhone into my Houzz on my iPad. How do I do this?

Hi @psinva, I believe the buttons mentioned above are at web, therefore you could login houzz through browser. In the upper right corner of ideabook view, you could find the organize option (See screenshot below). After clicking it, you could move any items to your other ideabooks:

It does have the same functionality at app, but it's called "Select" at the upper right corner of ideabook view.