"Is the online documentation not good enough?"

No not really.

For me the issue is also the depth of information out there.
The postgres online documentation usualy is the only resource. Need an in depth explination on how some part of postgres works, it is really hard to find.

With a book i can browse thru and pick up valuable information "at random", something which is so much harder with online documentation.

Then for me is also the preception.
There is a plethora of Oracle resources. it is high profile. i can find thru the vast ammount of literature something that meets my needs.

MySQL is also high profile. lots of resources.

With Postgres when i have been working with it i am struck immediately by the dearth of information. I know from personal experience that it is a powerful database, but the lack of information available, give me an impression on not being professional.

well, just some thoughts.