If I read a 300 page book and it has even just one sentence that somehow speaks to me and makes me introspect or reconsider or change some mental model I have, I absolutely find it with while. Sometimes the one encounter is enough to cause change (though sometimes it takes a while for my brain to change/adapt), and sometimes it takes multiple encounters with the idea.

IMO any book can be categorized as "self-help" - most business books I read, psychology books, biographies, etc, I read to enjoy, and the best ones (the ones I tend to enjoy the most and remember) are those that I'm changed by (ie. improved) in some way.

People get too caught up in labeling things instead of trying it out and seeing it works for you. You can read (for example) a celebrity gossip magazine and finish with great personal insights - the probability is just low - but if you do, awesome! Same goes with "self-help" books.

I also think it's highly dependent on timing. A book one year may have no meaning to you in one moment but be life changing in a future moment. It's the difference between reading things applicable to your situation now vs future need - the former approach is generally much better.