Basically.you have to read it like 3 times.

The first time, you just read it normally. A lot of stuff probably won't make sense because it's explained in later chapters or expanded on elsewhere etc. So you're not going to understand anything by reading it slower.

After the first read, you should have an understanding of the general concepts and the sort of main point of the book. Usually there is a problem, a proposed solution, some arguments to support the solution and then a conclusion or wrap up of it all.

Then you read it a second time, more carefully now, with the knowledge of your first read. This time, you should be able to understand better how the author is linking various ideas, identify some threads that continue through the chapters and how the arguments are built.

With the second read you should have a good idea of the problem, how the arguments are structured and how they build together to support whatever the author is trying to say.

In the third read (which tbh, is optional unless you really want to get to the nitty gritty and truly understand) you should read it and take note of the language, specific vocabulary the author uses for concepts, more critically look at the arguments and whether they make sense etc.

I'll just add some additional stuff in case it comes useful. This is a good way to read academic papers etc. You take advantage of the fact that they're structured the same, so if you're looking for relevant research, you can skim a bunch of papers because you know where the problem, arguments, methods, conclusion etc are. And if it's not relevant to you you can move on without needing to read an irrelevant paper in detail.

Once you recognise that certain types of books, papers, journal articles etc. All have a specific structure and purpose, it becomes a lot easier to extract information from them at whatever level of detail you want.

EDIT: regrading your comment that you remember the main idea but missing something, it sounds like you should pay more attention to the arguments. You can flip to the last chapter and get the main idea, but the actual meat and potatoes is actually the arguments of how they came to that idea. For example many philosophers are often summarised by some pithy sentence like "life is pointless" but the core is actually all the work they did to set up their final idea.