TAPI at MSDN online

http://msdn.microsoft.com/library/en-us/tapi/tapi3/telephony_application_programming_interfaces.asp contains documentation and specification

Andreas Marschall’s Toto® Tools

http://www.i-b-a-m.de/Andreas_Marschall%27s_Toto_Tools.htm is a collection of free, mostly TAPI related tools

ExceleTel Inc

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.