Capture Planning offers tutorials and workbooks for business and professional development.

Here is a list of free articles they offer:

Proposal Writing - How to Write a Business Proposal - How to Write an Executive Summary - Proposal Writing for Professional Services - Proposal Management - Red Teams & Proposal Quality Validation - Proposal Process & Procedures - Proposal Training - Business Proposal Software - Business Proposal Tips - Business Proposal Graphics - Oral Proposals and Presentations - Sales Letters & Copy Writing - Government Contracting - Request for Proposals (RFP) - Small Business Innovation Research (SBIR)