'Always on top' window on macOS

I need a way to put any chosen window to be always on top.

For example, if I read a book in iBooks or I'm writing code in Xcode using Sublime Text. I want an application or way to overcome the issue.

I tried Helium and Fluid Browser but they are like web browser it's not I look for. I exactly want any application to be on top always for macOS.

Try Deskovery3. It does that and more. (up to and including Catalina)

There's also Afloat (up to Mojave): http://www.perfectlyrandom.org/2016/10/23/always-on-top-in-macos-sierra/

Or AfloatX (for Catalina): https://github.com/jslegendre/AfloatX

Note: these require you to temporarily lower the security settings on your Mac (disabling SIP) during part of the installation/setup.

Alternatively, you might want to investigate the following macOS features: Full Screen, Mission Control, Hot Corners and Spaces which all allow easy and flexible control of application windows.