Hi, Claude -
Just a couple of comments for you.
In general, I like the designs, but there are a couple of things you can do to improve them...
If there is a "trick" to designing ebook interiors, it's to not let the design
be the focal point of the ebook. The design should enhance
the ebook, not take away from it. It should not distract the reader from the words of the ebook.
Colors should be less intense - for example, backgrounds more faded, illustrations recognizable, but less emphasized. The type within the illustrations should be clear if it's navigational type (some of yours is blurry).
Heavy black type should have minimal shadowing (see the web gold example)- it will make the type easier to read. Gold should NEVER be used for any kind of body copy, or Table of Contents - it's just too hard to read. Try a darker shade.(It's OK with the shadowing in the larger headlines, though you could use a slightly darker shadow with light color type to make the type stand out more.)
The images behind the titles, down the left side and at the bottom are much too "heavy"! Fade them out to 10 or 15% and it will look much better. Keep the nav buttons at 100%, though, to make them stand out, they're lost in there right now. (The best way to do this is to use "layering" where you can give each layer a different percentage value.)
All-in-all, it's a good first effort. Let me know if there's anything I can do to help you.