Which hook to use for updates of all nodes of a particular type This question is about Drupal 7.
For a particular node type, let's take three examples:


a) book
b) article
c) customcontenttype


I would like to be able to capture the moment of an update of an existing node of this type. 

I would like to know if there is a way to target only that node type directly from the hook and not to have the hook triggered for all nodes and then checking the node type.

I've seen some internet articles talking about 


mymodule_node_update - for me this is a general hook for all
nodes
mymodule_book_update or mymodule_article_update or
mymodule_customcontentype_update
mymodule_form_book_node_form_alter or
mymodule_form_article_node_form_alter or
mymodule_form_customcontenttype_node_form_alter


Which one of the three above hooks should I use? all of them? if none of them are good then which hook to use?

Many thanks
I would like to know if there is a way to target only that node type directly from the hook and not to have the hook triggered for all nodes and then checking the node type.


Node hooks aren't that granular, they're invoked for all nodes and there isn't a system in place to make that selective based on the type. If you're worried about performance, don't be; the difference between the hook being invoked for all nodes, and a system to allow it to be selective would be negligible.

If you want to react to existing nodes being update, hook_node_update() is definitely the right choice.

You just need to test the type in that function:

function MYMODULE_node_update($node) {
  if (in_array($node-&gt;type, ['book', 'article', ...])) {
    // Do something.
  }
}