Does your feature request involve difficulty for you to complete a task? Please describe.

I have the feeling all features have in a mind the library vision and not the "reader" vision.
By library, I talk about imported ebooks shown in main window and by reader, I talk about the way to use any file from the OS through the file explorer, like double click action from MS Windows file explorer

2 examples :

Example 1:
last added feature to allow multiple instance by enabling "open book in new window" work well when opening two imported books, the main library window stay opened while the 2 ebooks opened in separate windows. But when there is no Readest instance opened and we double click an ebook file in OS file explorer, the first ebook opened in a readest window, it's ok. When opening another ebook from OS file explorer, second ebook is not opened like expected and instead, the main library window is opened. Same for all other ebook opened from OS file explorer.
Those example work with "open last ebook on start" toggled to OFF

Example 2:
toggling the option "open last ebook on start" to ON and "open book in new window" to OFF
when opening a library imported ebook, then closing the readest window, then reopening readest => last opened ebook is reopened like expected.
Closing all readest windows, double click on an ebook from OS file explorer => the chosen ebook is not opened and instead, the previously opened imported in library ebook is opened => not expecting that

Describe the solution you'd like

Same behavior when an ebook is opened from any source (library, OS file explorer) for feature like "open book in new window" or "open last ebook on start"

Additional context

Using OS Windows 10 x64 and having epub opened by default in Readest
To understand better my concerns : I have lot of ebook, I don't care about library, cloud storage, and want a reader opening big ebook fast with correct formating (not like Sumatra) with a modern interface (not very the case of calibre). Library import extract the epub content elsewhere, I dont want all ebook goes in this location, so just open is the most important feature, library should be an extra, not the central place to work with the ebook.
I like libraries like Winamp or AIMP : just indexing files in some folders to fast search access, but not extracting the epub content (like Readest or NeatReader) or duplicating the files (like Calibre or Adobe Digital Editions does).