I run an online food blog. In the website, I post modified recipes that I find, I improve them, and post them; with credit of course. Does this count as plagiarism? I want to make an eBook, is it sufficient to list just the sources or do I need written approval from them?
Plagiarism is an academic concept, not a legal one. The issue you are concerned about is copyright infringement.
A work based off of another copyrighted work is a derivative work. This requires permission of the copyright holder. Simply listing your source and saying "no crime intended" does not help.
However, recipes are generally not copyrightable (at least in the UK/US). This is because they are a list of ingredients and instructions which is not sufficient for a creative work. The presentation of the recipe can be copyrighted, as can any descriptive text surrounding it.
If you're just taking the actual recipe, making a better recipe from it, and presenting that recipe in a better way, you should be fine.