Because Joomla has been used on thousands of sites, there is a huge amount of potential help out there on the Internet. A quick search on Google will reveal lots, but it is important to ensure that the help you are using refers to the version of Joomla that you are using. Comfycomputing's help sheets are of course 100% reliable (!) You also need to bear in mind that very often the help documentation is written in English by a non-native speaker which can cause some confusion.
As most of my clients using Joomla have to pay for technical support, I can understand my being used only as a last resort. My recommended alternative sources are, in order of reliability,
1. Documentation included in Joomla: press the help button
2. Discussion threads on the Joomla Forum (forum.joomla.org)
3. Other Joomla dedicated forums (search Google Joomla 2.5 help)
4 YouTube videos can sometimes be helpful
5 Google results please ensure you use 2.5 as one of your search terms
If you do have a problem resolved by use of one or all of the above sites, do please let me know and I will try to add a general article about the topic was creating problems for the benefit of other clients.
Joomla Books:I have looked in depth at books relating to Joomla 1.5, but not Joomla 2, and consequently I am not able to offer any firm recommendations, however the followingPDF looks promising, if rather dry.
If you are interested in buying a book I would suggest that you first check very carefully that it relates to Joomla 2.5, and secondly that it is more than just a list of contents and a reworking of text screens. Additionally please bear in mind that many of the books published immediately after release of a new Joomla version are in fact based on draft proposals and amendments which undergo further metamorphosis before being finalised. So really it is a case of caveat emptor.