DeutschSoftware developer Tom Hombergs recently wrote an article about making log statements more useful by providing context in order to make it easier for a developer who uses your libraries or builds upon your code to find out what the actual cause of an error is.
The blog post contains many pertinent examples of both good and bad log output.
Seemingly trivial improvements such as making your log output more helpful and explicit can make a huge difference and save a lot of time and effort.