Have you seen any menu bar designs online that you really like? Often, it's best to draw inspiration from what's already been done - take note of colours, font and overall layout and try to recreate elements that appeal to you.
For resources, CodePen and GitHub are great places to find menu bar designs and snippets of code to play around with and adapt. You could also check out some UI libraries like Bootstrap or UIKit which have pre-built components that you can implement.
It might be useful to know what tech stack you're working with - is this a purely front-end project, and do you have any specific frameworks/languages you're using? That might help pinpoint some specific resources!
As for general feedback - an online book club is a lovely concept! Have you thought about incorporating some kind of community element on the site, to encourage interaction and discussion between 'members'? This could add real value and a unique twist to your site, bringing people together not just around books but also around the actual act of discussing them.
It's a very fun project you've got going - good luck!