As someone who reads the Sims 2 manual is for pleasure, I cannot believe that I am going to give the Sims 3 manual only three stars, but that is all that deserves.
It starts out well enough, giving a sufficient overview of some of the major changes in this edition, hinting at traits and moodlets as being important, giving me an example of two weeks of a couple's life, before going into some detail about the traits. However, the level of detail in the previous version is on the lines of "can learn charisma at 30% faster rate"; here it is "can read faster". It gives some detail, but not sufficient for somebody who is actually bought a book about a computer game.
It then goes downhill with the wishes table. This shows every single and wish in this game over 20 pages, with the name and description, but it is not obviously organised. However, the description is probably no more than shown in the game, and it is not in any order; "eat stu surprise" is in five separate times on four different pages, and is only repeated because the ingredients are different.
You can tell that this book was being developed while the game was still going through some final changes, because in the moodlets table, the different moods are shown in order of severity, except this isn't quite true; obviously, in fine tweaking some of the moods changed severity, but it didn't change their place on the table.
Also, there is a lack of a final read through this document; for instance, it is said explicitly that there are seven commodities, whereas the table clearly shows eight.
Overall, the game may not be disappointing, but this manual certainly is. So, should you buy it? If you want an easy to follow guide of careers, traits, and the broader mechanisms underlaying this game - yes. If you want good quality detail of something that is not obvious whilst playing the game - no.