How do I Transfer Books From Calibre to iPad

Hi.
Can someone point me to the appropriate guidance please?

I am using Calibre 3.32 on an IMAC running IOS 10.13.6
I have connected my iPad Pro running IOS 12.0. The connection is via USB.
I have made sure the iPAD is the ONLY device connected (an iPhone and another iPad were connected via WiFi, but no longer)
I have next started Calibre in the iMAC and selected the book I want to put on the iPAD

I no longer see 'Transfer To Device' icon as I once used to.

I'm sure I have seen guidance somewhere before but can no longer find it. Pointers to it would be really handy - or the answer - perhaps as a 'sticky' somewhere?

:thanks: