Book Creator uses your device and browser's settings to provide spelling and grammar capabilities. So the first thing to do is to check browser or device settings to see what options are available to you.
For example, if you're using Book Creator online in the Chrome browser, any words that are mis-spelt will be underlined in red in the text editor. On some devices, you can right-click on the word and you'll be offered an alternative.
Note: words will not be underlined when you leave the text editor and view the text on the page.
If you're using an iPad, you can turn on spell check and predictive text at Settings > General > Keyboard. You can also turn on other useful typing aids such as Auto-Capitalisation, Auto-Correction and Smart Punctuation.