It looks like the text on the book now buttons have text using a cufon font embedding, so there is probably a way to edit it without having to create new graphics for the buttons, however I can't see how the buttons are being generated as there are not many clues to go on when viewing the source and it doesn't appear to use a theme or framework I recognise.
These sections on the homepage look as if they are pulled in as excerpts of posts or maybe custom post/content types, so there may be separate entries for them listed under posts (in a particular category) or possibly under another menu item - so there's ptobably nothing you can edit on the 'home' page that will change them. The buttons could be built on those posts as inline html or there may be a value in a custom field or custom meta box. There could also be template file in the theme that might need to be edited. I'm afraid the way to find this out would be to access the site files by ftp and login to admin and work it out. Someone experienced with WordPress will probably be able to work it out quite quickly, but if you are at the bottom of the WordPress learning curve, it puts you at a bit of a disadvantage.
If you are stuck, you may need to get your boss to agree to pay for an experienced WordPress developer to log in and look at the code and produce some documentation for you - or make the changes if it is a one-off job. You should be able to find someone at http://jobs.wordpress.net