So there are databases out there, but at least for a start you can implement a fairly effective paperwork
system using Office. If you creating a booking form, electronic or paper, (in many ways paper is still king here) which lists the client,
venue and their requirements.
From there I'd put it into the calendar in Outlook, so that you can open up a given day and see what's on, what's available, what's dark.
Then create a spreadsheet in Excel with assets down the rows and dates across the columns and "
book" the gear to a job by say job number (create a unique one per booking and reference it in all paperwork). Create another sheet within the same
book with people substituted for assets and you should start to be in a position where you have some control.
Of course all that could be integrated into an access database or the like if you have the skillset and time required...