Current generation Nextion displays can have multiple pages on board with no SD card required. Not sure what the limit is - probably varies by model. The Arduino library for Nextion is 100% open source - but, as far as I know, not the Nextion firmware. Its a good thing the library is open source because they don't seem to be actively maintaining it. One of the guys on Arduino.cc has rewritten the hardware module to fix some serious serial programming problems. Other than that I find the core library easy to use and reliable so far. And since there is on-board processing and scripting, if you have kept the GUI and the business logic sufficiently independent you can offload a fair amount of work to the Nextion. I don't have experience with any other MCU based displays. I've now written for LCD, display only TFT, and Nextion Touch screen. Is there a good IDE for building touch TFT displays? At around $35 for the 3.5" display the Nextion is still a bit pricy. Are there alternative MCU based displays that are more price competitive?
Looking forward to your books. Even though I have been programming for 40 years, I don't do it often enough anymore that so sin-tax is rusty - I know that a thing can be done and usually remember how to do it - only to find that the compiler shouts at me for being old and using deprecated features or syntax. And its always good have well written, properly formed examples to plagiarize. I've often commented that the only truly original program is "Hello world" - everything else is a copy with slight modifications.