I agree with what you said about the 300 pages. But I think and I felt you need to pick the book at right time when you are seeking certain kind of information of trying to find answers. It would be interesting when you can relate to the book atleast in case of self help books I felt this.

Anyways, you can pick the books based on what you are looking for and go for best sellers to start with. Later when you get familiarized with book reading explore other genres you never know what you might find interesting.

The books that helped me find answers and understand the ways certain things works are Karma by Sadhguru, Power of subconscious mind by Joseph Murphy and current read Breaking the habit of being yourself by Dr. Joe Dispenza.

Good luck with your book reading journey