This Lease Book (which is held in the private library of Knowsley Hall) is the main source of information about Manx leases before 1643. I recently visited to view the manuscript and have since received permission to upload my transcription of it to my site (manxmanorialroll.com), as has now been done.

The document itself only includes the names of a minority of tenants (there were about 340 leases agreed in 1630, and rather fewer before that). The earliest dates from 1542.

This is a source that hasn't been publicly available before, and would therefore be worth checking for those with an interest in the period.