Hi Ahmed,
welcome to the forum!
There are no books I'm aware of, if there were, they'd be huge - Wappler is a very capable piece of software, and as you've seen is quite complex. The creators have done a good job in making it as easy as possible to generate great code but there is still a bit of a steep learning curve.
There's a great breakdown of what you need to know here, some if it is dated but the principles are mostly the same across the different Wappler versions:
Theres also a list of up to date videos here:
Sometimes the best way to learn is to come up with an idea of what you'd like to create.
Once you have a basic idea, break it down into stages, then start off small and keep going until you hit a problem, when that happens, use the search function on this forum (most of your mistakes will already have been experience by someone here). If you can't find an answer, create a help post here, be as descriptive as you can, and someone will pitch in to help.
You'll probably find it quite frustrating when you are starting out, as there are so many unknowns, but with a little perseverance it really wont take long before you start to see some great progress.