a book file is not an InDesign document where you work with objects on your pages.
Consider a InDesign book file as a platform for organizing and synchronizing several InDesign documents.
It will not help renaming the suffix. Not at all. It will remain a book file.
A possible scenario what could have happened to you:
If a *.indb is opened and saved in a higher version of InDesign you cannot open it in a lower version.
Unfortunately there is no conversion mechanism, no save-back mechanism, like the IDML export, to save it in an exchange format that one can open it in more than one version.
You did your *.indb file in InDesign CC 2014, added and synchronized some InDesign documents. Saved and closed.
Then you opened the *.indb file in CC 2015. Perhaps you did some changes, maybe you updated the list of InDesign documents or you simply saved the indb file because its state was "changed".
Now back to CC 2014 you are not able to open the indb file in that version.
Create a new indb file from scratch in the version of InDesign you are currently working with.
What is, what are the exact version/s of InDesign that are installed on your machine?
Start InDesign and check the menu "About InDesign".
What is your exact version number of your operating system?
Hope, that helps.
Thank you for your information. It's good to know for the future. After I tried to
Open the file following some recommendations without any success and after cried for a little bit I start a new project .....
Thank you very much...
I use inDesign CC and Yosemite operative system but I don't know the version ... ( I m not working now, it's 12:09 here )
Sent from my iPhone
You might not have noticed the .indb opening -- it would open as a panel listing the component .indd files, which you can open from there. You must have had at least one .indd file included in the Book as .indb can only hold pointers to .indd files and no actual work takes place on that file.
I suspect you may have started by using File > New > Book... instead of File > New > Document... if you are working on a long project with more than one chapter. As Uwe mentioned, to InDesign "Book" means a collection of individual documents organized to output as a single unit, regardless of the content. A Document is a single layout file, of any length, and does not imply the content type. It can be a simple advertisement, or a 1000-page novel.