Syncfusion (a well known company that provides enterprise-class software components and tools for the Microsoft .NET platform) has published on its Technology Resource Portal a collection of 28 free e-books about several technologies, languages and frameworks. All of them include the “Succinctly” term in the title, which called my attention because it means that they are larger that an article or blog post, but smaller than the typical book of 400-500 pages. Interestingly enough, one of them is about Visual Studio add-ins:
You only need to provide your name, company, e-mail and phone to download them. Even better, they are provided in .pdf and Kindle formats.