The phrase "pledge your first-born child to" is a sarcastic figure of speech, akin to "sell your soul to". It's saying that someone is asking you to pay too high a price for something, and is often used when you might be getting yourself into a situation that could be hard to get out of later.
In the context of technology, it's often used to say that you'll be tied to a certain platform or system, and switching might be difficult or costly.
For example, say a mobile phone company is offering you a free phone, but you'll need to sign a two-year contract to get it. Some people might think that's too long to commit to one carrier, particularly if you could buy the phone yourself at a reasonable price. In that case, the person who is advising you to reconsider that "deal" might say something like:
Sure, you get a free phone – but, to get it, you have to pledge your first-born child to the carrier.
or, similarly:
Sure, you get a free phone – but you have to sell your soul to get it.
It's very figurative speech to say you might regret something down the road.
It is a common knowledge that animals pee or poop (excrete) when they are frightened to death. People do, too. Also, animals do it right before their death. That's where the expression came from.
If you scare the s*** out of someone, it figuratively means you scare someone to the extent that they would urinate or defecate. It is a very common expression.
Note: Some do use scare the pee/urine out of someone. It has the same meaning, but less idiomatic and popular.
Sentences such as "I extremely scared her" or "I scared her very much" would not work very well because there is a more common and idiomatic expression.
The expression evolved to become an intensifier for a verb.
"Let's bomb its oil field" doesn't sound intense enough, however, "Let's bomb the s*** out of its oil field" sounds very intense. You just metaphorically compare "its oil field" to an animal or human.
As explained in the other answer, the list of expressions goes on and on. The most important thing to remember is:
Whatever expletive is put in X in Verb X out of Y constructions, it is a strong
intensifier. It gets the message across in a strong/intense sense.
I will stop writing on this now before some members "criticize the s*** out of me".
Note: You can use it for a verb like criticize, which means you can be creative in using it.
Edit:
As commented below, the above expression can be toned down a little by saying, "Bomb the hell out of its oil field". It's slightly more acceptable than "bomb the s*** out of its oil field".
Best Answer
The point of the joke is that the warning comes too late - the first person has already fallen before his friend tells him to watch out ("it's a doozy" simply means "it's a big one" or "it's a deep one"). There are several variations on the joke, such as when someone bangs their head on a low door frame and the person behind them says "watch your head" after the fact.
In the context of the quote, it simply means that many people will buy a book on networking without first making sure it's at a level they understand - they've taken a step without realizing how deep it really is.