While it depends on the exact requirements, I'd generally recommend Commerce
instead of SimpleCart these days.
Commerce is officially in beta, but ready to use in production if it has the features you need. We still want to add things like invoicing and sale pricing before calling it 1.0, hopefully, this summer/autumn.
One important distinction between SimpleCart and Commerce, is that SimpleCart was born out of a specific webshop, which was then made to support other use cases and released as an extra. Commerce was built from scratch to be usable as a platform for a much wider array of e-commerce use cases that can be extended and built upon. Many of the core features in Commerce (like coupons and address validation) are added as modules that can be enabled/disabled/extended without getting in the way of keeping your site upgraded.
We're also starting to see community-built extensions for Commerce getting released in the past few months, which I think is quite exciting to see happening
For a book store with some merchandise, SimpleCart can probably work just fine. Some features that would push you into Commerce would be flexible shipping prices
(SimpleCart supports only fixed price options with "free from $X", Commerce has weight/country/tablerates and the possibility for fulfillment API integrations), payment gateways (Commerce has all the gateways SimpleCart has, and then some
), and more powerful taxation rules including a "prices inclusive of VAT" mode.
MiniShop2 is also a very capable solution, which has the benefit of a very large community and extensive list of available extensions. The primary things it lacks is taxes and while there are a ton of payment gateways available, common western gateways are missing. It's cataloging features are impressive though
Not sure what happened with BluCart. It looked interesting at the time, and FoxyCart (these days just called "Foxy") is a very mature and powerful platform to integrate with, but I don't recall ever seeing BlueCart getting publicly released. This forum post
suggests it's no longer being worked on.