Pressbooks
Based on Wordpress, Pressbooks is an ebook creation tool, which also provides a number of helpful features for creating a textbook, including automatically generated front and back matter, a collection of themes for easily modifying the appearance, LaTeX support, and hosting on Pressbooks.
What you will need: Pressbooks account. With the free account, your finished book will contain a watermark on each page, but the paid version will get you watermark-free files. The web version of your book will also be private, unless you upgrade your account.
When to Use It: If you're creating an open textbook, Pressbooks is one of the best tools available--but note that you will have to pay a fee in order to get files that you can share.
Gitbooks
Created by GitHub, GitBooks is an open source tool that allows you to create a textbook that is hosted in a GitHub repository. You can create your content in Markdown or embed rich, multimedia content. There is currently no PDF export option. This tool was originally developed for creating technical documentation guides, so it does not have as many of the features of other OER tools.
What you will need: Free account with Gitbook (or GitHub). There are some limitations on number of collaborators and spaces for the free account.
When to Use It: If you're creating an open textbook, are familiar with GitHub, and want to use the features provided by GitHub (versioning, collaboration, etc.) but in a "book" format
Bookdown
Bookdown is an open source R package that allows you to write books and long-form articles/reports with R Markdown. Bookdown supports a wide range of programming languages, as well as graphics and interactive applications. You can export your content in multiple formats: PDF, LaTeX, HTML, EPUB, and Word. Can be published to GitHub, bookdown.org, or any web server.
What you will need: Free account with GitHub, R Markdown and associated software packages
When to Use It: If you're creating an open textbook, are familiar with R and Markdown, and want an open source solution
JupyterNotebook
Jupyter Notebook is an open source web application that allows you to create and share documents that contain live code, equations, visualizations, and text. Jupyter supports over 40 programming languages and can be shared through GitHub or the Jupyter Notebook Viewer.
What you will need: Python 3, Jupyter (Anaconda can be used for installation)
When to Use It: If you're creating a computational OER that involves users interacting with code or visualizations, and you want an open source solution