Just an idea:
wouldn't CopyFromAndSave(source As %Stream.Object) be a better approach ??

just after

Do Httprequest.EntityBody.Write(mystring)
Do Httprequest.EntityBody.%Save()

As with any other object

Once again I fail to follow the repeated almost religious secret of $LIST() encoding
that is celebrated by Support and Engineering over decades.
Especially when the use of the knowledge is labeled "illegal".
That's just disappointing.

The problem of a possible unexpected change rather indicates incomplete Release Notes to me. 

Status= 1 justteslls you tht the HTTP connection was processed OK.
Next you need to take a look into your response object.

set res=Httprequest.HttpResponse
in terminal then ZW res to see in the Status is 200 OK
otherwise what you see I  res.Data might be just an error page.
Which is also OK for HTTP but not for your content.

• property ReasonPhrase as %String;

This is the human readable reason that goes with the StatusCode.

• property StatusCode as %Integer;

The HTTP status code. This is useful to determine if the request was successful. Look in the rfc for HTTP to see which codes are supported and what they mean. A human readable form of this code is stored as the ReasonPhrase

• property StatusLine as %String;

The HTTP status line. This is the first line of the response and signals if the request was successful or if there was a problem.

I found %DynamicObjects first in Caché 2017.1 Class Ref
https://cedocs.intersystems.com/ens20171/csp/documatic/%25CSP.Documatic.cls?&LIBRARY=%25SYS&CLASSNAME=%25Library.DynamicObject
In 2014 this was somehow buried in %ZAN.Auxilary.???

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.

the story is clear.  Your class definition is not.
once more pls show class definitions for RESTAPI.TITLE   and    ??.BOOK   
(every class has also a package, default is User) 
you may need Studio od VSCode  to see it.

and {"ID":17, "Title":"LEARNING CAHE REST API"} is a
Dynamic JSON object unrelated to any class definition

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 withoutID but having RowID" ???

How did you get the screenshot?

Ah thanks!  That passed me.

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.  

• so your obj is a DynamicObject and looks something like { "ID":17, "Title": 44 }
• class All.Allbooks has this Property Title As User.Book. 
• To set it you need an oref !!

therefore to create the required oref :

SET task.Title = ##class(User.Book).%OpenId(obj.Title)