(I'm working on an update to this book which will cover building Shopify apps with .NET Core and Asp.Net Core! You'll get the update for free as soon as it's available.)
Building an app for the Shopify store is hard. Really hard. There are a ton of API calls, redirection URLs, webhooks and best practices that you'll need to string together just to get something that works. There's not one single place, website or resource that will show you all of the steps you'll need to take to build a rock-solid Shopify app. If you do find one, it will undoubtedly use Ruby on Rails, which can sometimes feel like you're reading moon runes if you're a C# and ASP.NET developer.
That's why I wrote The Shopify Development Handbook -- to distill my own experience of building Shopify applications into one concise and comprehensive course for ASP.NET developers.
In this guide, we'll go farther and deeper than Shopify's own documentation. We're going to build up a basic Shopify application that will connect your users' stores and sign them up for a monthly subscription to your app. Then, we're going to use that basic app as the basis for three more advanced Shopify apps.