Good evening, Woodelf.
It seems to be working OK for me.
If, by the 'combo box' you mean the box at the top left, I wonder whether your screen reader is handling it correctly. Clicking on the word 'Books', for example, doesn't actually do anything. Placing the mouse cursor over that word opens up a sub-menu to the right of the main box, with links to 'Books', 'Kindle eBooks'), 'Books for Study', etc.
Perhaps it's another example of a website which hasn't been designed with the needs of those with visual impairment in mind?
The drop-down menu, to the right of the word 'Search', enables you to select a particular department to search within (by entering your search term in the field to the right of that menu). Perhaps that might be of more use to you?
Chris