http://msdn.microsoft.com/library/en-us/tapi/tapi3/telephony_application_programming_interfaces.asp contains documentation and specification
http://www.i-b-a-m.de/Andreas_Marschall%27s_Toto_Tools.htm is a collection of free, mostly TAPI related tools
At http://www.exceletel.com you will find the well regarded and long running TeleTools series of developer tools along with related products, TeleTools and TAPI support resources, and access to both email and phone support, consulting, and development resources.