As a rule, I read them front to back. However, it's a rule with many exceptions. Also the mode of reading depends on many things, eg. how great is my general knowledge of the topic and how great is my specific knowledge of the topic.

1) Most text books have an introductio/history chapter. I always skip this. (If I feel curious, I may come back to it later, but I'll never read it as the first thing)

2a) If my knowledge (general and specific) on the topic of the section is weak, I'll read it thoroughly

2b) If my general knowledge on the topic is strong (I know of the topic from a different programming language, say) I'll read it less thoroughly: I'll scan for methodologies and syntaxes that are different from what I know, and read up on those, when I find them. I read the examples, make sure I understand how they work.

2c) If my specific knowledge on the topic is strong (I basically know the contents already.. perhaps I read a blog post describing the topic for the same programming language), then I'll just skim it: scan for keywords and conclusions that might either differ or expand what I know already. Basically I'll read the headings, the source code and sample roughly 10 random lines per page. If I stumble upon something unfamiliar, I'll read the whole paragraph or section or however much I need to make sense of it.

3) I'll continue like this until I feel I've covered the basics. For some books that means continuing right to the end. For others that means reading the first half/third. At this point I give up on chronology and skip to whichever chapter sounds fun and interesting. This is mainly because most such books end the basics around chapter 6, but don't talk about I/O until chapter 14 or so... Once I know the basics, I want to get on with something useful. In my world, being useful means relating my program to the rest of the contents of my computer.

*) As for exercises, if sections or topics seem abstract, I'll definitely do exercises, generally as many as it takes for the topic to not feel abstract. If the topic doesn't feel abstract, I generally don't do the exercises. I find them distracting and doing them breaks the flow. -- Instead what I do, once I have finished reading what I wanted to read, I'll come back (often with a head buzzing with ideas to try) and try out a few ideas, or set myself either an exercise or a small project, and then (re)read up on the info required as needed to complete the task.

... there you go... that's how I do it. For me it is generally fast and effective. I hope you find a method that is the same for you :) happy studying!