Edit book: Add a tool to check spellings, access it via Tools->Check spelling
The calibre book editor now has builtin spellcheck. You can install new dictionaries via Preferences->Editor->Manage spelling dictionaries. It comes with builtin dictionaries for English and Spanish. You can add OpenOffice dictionaries (in .oxt format). The tool checks spellings in all HTML/OPF/NCX files in the book, taking into account any language declarations in the book, so that it will work correctly on multi-lingual books as well. The code is very new, so there may well be bugs. As you type spell checking is not yet implemented.