Where Self-plagiarism Begins and Where it Ends Academics sometimes publish popular books based on their own research, but not always cite properly themselves, even when book references other authors properly. Moreover, sometimes I can see that one author publishes very similar (but not same) papers on the same topic, without self-referencing, even though it can be seen that the author recycles some of hers/his ideas. Even respectable people in the profession do it. 

Given this where is the line beyond which we are talking about self-plagiarism? 

On one hand it feels bit wrong to do this even in a small amount. On the other hand I can imagine that it can be impractical, and cumbersome to piously self-reference yourself, and I also worry that that might seem egocentric to the reader. 

It is no brainier that copying part of your old article, and submitting it as new is clearly wrong. On the other hand it does not seems to be wrong using the main ideas, quotes (or chapter titles), or paraphrasing minor ideas of your earlier papers to publish a book without using the same referencing standards for yourself as you use for other authors.

Any opinion on this? Should I always reference myself as well as I do others, or is it ok to be more lenient on yourself as long as it is not more excessive then it is usual in the field? Do you think that this depends mostly on the context?
Let us set aside debate about whether the word "self-plagiarism" makes logical sense. The concept is fairly generally acknowledged, and rests on a simple ethical principle: it is wrong to give the impression that a work is novel when it is not.

We can further distinguish two distinct forms of self-plagiarism:


Literal self-plagiarism, in which words are directly reused.
Conceptual self-plagiarism, in which the words are different but the content is (largely) the same.


From this definition, we can elaborate several key points of evaluating whether a reuse counts as self-plagiarism:


Is there an expectation of novelty (e.g., a journal article, conference paper, or book)?  In some cases, such as one-paragraph conference abstracts in some fields, there may not be an expectation of novelty, in which case there is no "impression of novelty" to be considered and self-plagiarism is not at issue (the reviewers may still decide they are uninterested if the work is insufficiently novel).
Is more than a trivial amount of material being directly reused?  If so, then they must either be in a quotation, or else the publication must be clear that portions have previously been published, and a citation to the prior publication must be included.  Otherwise, it is self-plagiarism. (Note that this may not apply to "non-creative" words, such as a tersely written methods section).
If concepts are shared with prior publication, then there must be clear attribution of the significant antecedents.  There is a lot of flexibility that enters with "significant," and different communities interpret this in different ways, so you need to know your community expectations.  Here, one basically treats one's own results just like those of any other scientist, with the exception that one cannot claim ignorance of one's own prior publications.