Over the years I've been asked many times, what training did you take or books you read to learn how to develop add-ons. For me there was no real formal training other than Autodesk University. I have learned to develop add-ons through reading various blogs and online resources.
Below is a list of those resources I have used over the years so that others may be able to find help as well.
Visual Studio
Information on how to get started, first exercises, etc all on the left side.
https://help.autodesk.com/view/OARX/2022/ENU/?guid=GUID-71683E52-850F-434E-AD8C-3DB20BCBAD14
User Interfaces
WinForms
https://www.c-sharpcorner.com/article/tutorial-working-with-windows-forms-part-i/
https://www.pluralsight.com/courses/windows-forms-introduction-with-visual-basic
WPF
Several links I've used over the years to help learn WPF
https://www.c-sharpcorner.com/technologies/wpf
https://www.tutorialspoint.com/wpf/index.htm
Localization
https://www.autodesk.com/autodesk-university/class/Localizing-NET-Applications-AutoCAD-2019
Troubleshooting
I usually start with Google and type in what I'm attempting to do or the issue I'm having and search that way. When searching, I've found it most helpful to search by your product or UI depending on the issue (AutoCad, Civil 3D, WPF, WinForms), then I almost always use ".NET" in the search or will sometimes add VB.NET or C#.NET to clarify the language, and finally a string of what I'm attempting to do or issue.
Examples of troubleshooting searches:
ACAD .NET Create a line
ACAD VB.NET Get Text Styles
ACAD C#.NET Get Dimension Styles
C3D .NET Get Surface Elevation
WPF .NET Bind to a DataGrid
WinForms .NET Format DataGrid
Troubleshooting Helpful Sites
https://www.c-sharpcorner.com/
AutoCAD .NET Resources
Autodesk Developer's Guide
https://help.autodesk.com/view/OARX/2022/ENU/?guid=GUID-C1188931-AC7B-4B95-B5AE-11162607648E
ADN DevBlog
https://adndevblog.typepad.com/autocad/
Through the Interface
The Swamp
https://www.theswamp.org/index.php?PHPSESSID=b07d9d02ca433ebf59786a35b0ef490b&
Spider In Net Blog
https://spiderinnet1.typepad.com/blog/
Snoop AutoCAD Database Tool
https://github.com/chuongmep/AutoCADLookup
Autodesk University Classes
https://www.autodesk.com/autodesk-university/class/Plotting-and-Publishing-Drawings-NET-2012
https://www.autodesk.com/autodesk-university/class/Bridging-Gap-Extending-AutoLISP-NET-2021
https://www.autodesk.com/autodesk-university/class/Hands-Introduction-VBNET-Add-Ins-AutoCAD-2018
Civil 3D .NET Resources
Civil 3D Basics
Infrastructure Modeling DevBlog
https://adndevblog.typepad.com/infrastructure/
Civil 3D Reminders
http://blog.civil3dreminders.com/
Civilized Development
https://civilizeddevelopment.typepad.com/civilized-development/
The Building Coder
https://thebuildingcoder.typepad.com/
Snoop Civil 3D Database Tool
https://github.com/ADN-DevTech/Civil3DSnoop
I hope this is helpful for all who are looking to get into writing code for AutoCAD, Civil 3D, and other Autodesk products.