therefore I asked for a FULL class definition for BOOK!
But I got just 1 line.
there can't be help without sufficient information

Now, this gets clear.
With the keyword IDKEY you replaced the default ID naming it RowID.

To store it:

set book=##class(BOOK).%New()
set book.RowId=obj.ID     ;  from JSON obj
set book.Title=obj.Title    ;  from JSON obj
do book.%Save()

to retrieve an existing Rowid:

set book=##class(BOOK).%OpenId(obj.ID)   ;from JSON obj
,;; access or change your book.Title

I'm sorry. it seems you don't understand what I'm talking about.
You just gave me the names. Not the structure and definition. 

Expected example:

Class RestApi.Books Extends %Persistent
{
Property Title As %String;
Property Pages As %Integer; 

Storage Default
{
<Data name="BooksDefaultData">
<Value name="1">
<Value>%%CLASSNAME</Value>
</Value>
<Value name="2">
<Value>Title</Value>
</Value>
<Value name="3">
<Value>Pages</Value>
</Value>
</Data>
<DataLocation>^RestApi.BooksD</DataLocation>
<DefaultData>BooksDefaultData</DefaultData>
<IdLocation>^RestApi.BooksD</IdLocation>
<IndexLocation>^RestApi.BooksI</IndexLocation>
<StreamLocation>^RestApi.BooksS</StreamLocation>
}
}

 Class RestApi.Title Extends %Persistent
{
Property Title As Books;
Property Text As %String; 

Storage Default
{
<Data name="TitleDefaultData">
<Value name="1">
<Value>%%CLASSNAME</Value>
</Value>
<Value name="2">
<Value>Title</Value>
</Value>
<Value name="3">
<Value>Text</Value>
</Value>
</Data>
<DataLocation>^RestApi.TitleD</DataLocation>
<DefaultData>TitleDefaultData</DefaultData>
<IdLocation>^RestApi.TitleD</IdLocation>
<IndexLocation>^RestApi.TitleI</IndexLocation>
<StreamLocation>^RestApi.TitleS</StreamLocation>
}
}

it's midnight now. I finish 

maybe next week.

This returns to what we were talking about yesterday.

You mix classes with JSON objects and RowID might be something different.
so pls add the definition of the classes you talk about (e.g: RESTAPI.TITLE and  TITLE table ??)
it's not obvious what you refer to in "object without ID but having RowID" ???

How did you get the screenshot?

Assuming your RowId is unique you can define a unique Index on it.

Index RowIdx On RowId [ Type = index, Unique ];

now (after building that index) you can open your object by its RowID​​​​​​​

SET task.Title = ##class(RESTAPI.TITLE).RowIdxOpen(RowID)

Ah thanks!  That passed me.

Every Database in Caché or IRIS has a "NickName" (TEST, APP, USER, *TEMP, ..)
The file location is just a pointer related to the location of the file used
Similar Resource is an attribute to this "NickName".  and NOT to the related file
Changing it affects user access, but doesn't care at all about the file location.
The *.DAT file has no information about Resources or its NickName.  

Way back in the past, when file sizes were limited by file systems, there were
"Continuation" Files for Cache.DAT. Those knew the starting Cache.DAT.
But this is decades back and gone and doesn't exist anymore. 

To find this quoting in Windows kept me busy for quite a while 2 months ago
as it wasn't part of the README.md  !
 

GRAND MERCI !

Exactly! 
I just was not sure about the abbreviation.  

I will not be present, but my main interest is and ever was
Distributed Code and Version Management in a World Wide Scope  
I experienced it in personal practice and was always impressed how well that worked.

You depend on the structure of the dynamic object you compose in the 1st line.
if it has a property TitleID  it will work. (using obj.TitleID   lower case!!)

But in your question, it is not clear what Title Class refers to.
I see All.Allbooks and somewhere User.Book and also %request  and no other class around

For some of them exist even concrete proposals for solutions.  (e.g. Global <-> JSON) 

It should be the same port as SMP  -  default 52773

if this is inside a method  add [ProcedureBlock = 0]   to the method