I am looking for a complete online reference for iManage's latest API. On googling, can see references to iManage v2 documentation, but that seems to be bit old and where can I find latest API if at all there is one
The latest version which is constantly updated is available at https://help.imanage.com/hc/en-us/sections/360000214214-iManage-Work-REST-API-Documentation - you'll need a login to IManage's support site to access this though. If you don't have credentials you can raise a ticket with your support provider to get access.
If you have an on premises server that's licenced for the API, you'll find the documentation for that specific version at https://YOURSERVERNAME/api-docs/