Any suggestions for books / courses for a developer who wants to go into embedded development both professionally and as a hobby?
If you're looking for something to play with at the HW level, it can be overwhelming to know what to start with. Stick to Arm Cortex-M. That will get you the furthest the fastest IMHO. If I was to suggest one company -- even though they all have their strengths and weaknesses and niches, and I work regularly with several like NXP, ST, etc. -- I think Nordic Semiconductors does a very good job with their support forums, SDKs, tools, etc. BLE is also a very rewarding place to start since you can talk to your phone, laptop, etc., which is Nordic's niche.
A development board like the nRF5340 DK has a HW debugger on it out of the box (to program the MCU and debug programs), is reasonably priced, works cross platform, and packs a lot of processing and peripheral punch. Being based on the Cortex M33 it has a solid security foundation (TrustZone and TF-M), and works well with a first-class RTOS like Zephyr with open source networking and wireless stacks.
You'll find answers to common questions with this chip on their forums and online.
There are other options -- ST and NXP have many excellent MCUs and inexpensive dev boards -- but the nRF boards and the ecosystem around them make them a good choice to make a serious start and learning embedded, and they are one of the only vendors who reliably answer questions on their support forums. The nRF53 dev board brings a lot of value as a serious learning platform if you're getting started.
Again ... just an opinion!
Assuming you mean more writing firmware, the biggest thing to understand is that embedded is all about C. You'll absolutely want to learn the basics of C and properly understand pointers. A key part of C is understanding data types (signed, unsigned, float) and notations you rarely used in other fields like hexadecimal which is omnipresent in embedded. If you grew up learning C#, node, etc., you likely don't properly appreciate these fundamental types, and you'll need to learn those fundamentals, but that will come with learning C.
For books, I like Jack Ganssle's "The Art of Designing Embedded Systems". He does a good job of laying a solid foundation for planning embedded projects. It's opinionated, but you could do worse than start with his ideas.
And start with a professionally maintained foundation for your projects. Arduino is good for some people, but it won't scale and won't give you the skills you need professionally, and scripting languages like MicroPython won't help you later in life. Use a language (C) and platform you can go to production with, such as Zephyr RTOS, Azure RTOS, FreeRTOS, etc. It's more work and harder up front, but the investment will pay dividends and you'll learn good habits from the start.
Not because it's great technically and not because the editor is great (frankly, it's awful). The reason I argue to start there is that they've made the first 15 minute experience stupidly easy and convenient and, as a result, it's become wildly common and popular and you can readily find Arduino-platformed examples for most of the basic electronics technologies. If you're the type to learn best when you can see glimmers of visible progress, Arduino gives you smooth on-ramp.
You will need to wean yourself from that reliance/training wheels at some point, but I think it makes the first 2 months 20x easier, especially if you're trying to learn datatypes, bit-packing, pointers, memory management, analog electronics, digital electronics, communications protocols, in-circuit programming, and everything else (PCB design?) all at once. Break it up a little.
As long as you eventually take the training wheels off, and know that Arduino isn't a path that leads to being able to create financially viable products, and it's a first step.
It won't teach you certain good habits, but it will perhaps get you hooked and motivated, which is useful in itself, and does give you the satisfaction of making motors whirr and LEDs blink quickly.
I agree that Arduino is not a good start if you intend to become a professional. Arduino works really well for the non-technical person who will never progress beyond the arduino ecosystem, and for the experienced embedded programmer who is well aware of its limitations. Beyond that, if your intent is to learn embedded systems professionally, pick up an ST Discovery development system (about $20 IIRC) and have at it. Although I worked for a company that standardized its embedded development on Nordic processors, I don't recommend them unless you need wireless in your project.