Linking to other markdown files in the book's source generally works as expected. The exception are links to 'README.md' files. Clicking on such links in the HTML version of the book results in a 404 (Document not found) error.

Steps to reproduce:

1. Create a book's source as follows:

   src/book.toml:

   [book]
   authors = []
   language = "en"
   multilingual = false
   src = "src"

   src/SUMMARY.md:

   # Summary
   
   - [Chapter 1](./chapter_1/README.md)
   - [Chapter 2](./chapter_2/README.md)
       - [Chapter 2.1](./chapter_2/chapter_2.1.md)

   src/chapter_1/README.md:

   # Chapter 1
   
   - Link to [Chapter 2](../chapter_2/README.md)
   - Link to [Chapter 2.1](../chapter_2/chapter_2.1.md)

   src/chapter_2/README.md:

   # Chapter 2

   src/chapter_2/chapter_2.1.md:

   # Chapter 2.1

2. Build and serve the book: mdbook serve

3. Open the book in your browser: http://localhost:3000

4. Click on the links in Chapter 1: while the link to Chapter 2.1 works fine, the link to Chapter 2 doesn't.