What is the most feature rich "book recommendation" plugin?

Well first off I wouldn't recommend trying to power your client's database with the standard WordPress database instance. They can use WordPress for the website all they like. But with millions of records like that, you really don't want to move those data in with WordPress. For that matter, you're probably gonna want a dedicated database server to answer those questions, if the client doesn't already have one. You might also need to investigate something like Elastic Search, depending on what your search requirements are and what sort of database they are using (pay close attention to whether or not it provides full text search ability). Beyond that, and somewhat obviously, you're also gonna want SSL on the website and to make sure you're up to date on federal and state privacy requirements for data like this, as well as best practices for securely transferring data over the web (port configuration, ssh keys, etc). The privacy concerns would raise a flag for me. If those data have anything personally identifiable in them - which they very probably do - then you're really gonna want to make sure you do your research on security of personally identifiable information. Government specific privacy /security /auditability requirements may also apply, so double check that too.

We do have those people on Clarity. Jason comes to mind https://clarity.fm/asmartbear That being said, anyone on this list (who have software background) would be great https://clarity.fm/search/entrepreneurship

I've been down this road before (two custom CMS platforms built over the years), and to play devil's advocate here: do you really need to build a custom CMS? What problem are you up against that aren't being solved by existing solutions? If those problems aren't fundamental, how much time and money will be saved by creating plugins for an existing CMS? I know the temptation to start from scratch, but if this is something to be used in production, the todo list is staggering: you'll need user authentication, security on the front and back end, admin interfaces, plugin infrastructure for extensibility, theming (if this will be used for multiple clients), third party integrations — and that's all before you get into really annoying and notoriously finicky shit like media uploads and i18n. If it's for learning or a hobby, go nuts, but if you're planning to use this with clients, save yourself the heartache and late nights and use something established and open source. Join the community and help fix existing solutions' shortcomings — that's a bigger overall contribution to the world. Good luck!

You are comparing apples to oranges. WordPress is a platform, Ruby is a web framework. If you are looking to develop a completely custom platform, you could certainly do it in Rails, but you could also write it in PHP (like WordPress). I'd do some digging and see what is out there for pre-made software first before you build something from scratch. I'd imagine there are already plugins that would do this sort of thing for Expression Engine, Joomla or WordPress. No need to re-invent the wheel!

Checkout is a high-quality WordPress theme by Array built for marketplaces of any kind. https://arraythemes.com/themes/checkout-wordpress-theme/ Let me know if you have any questions about implementation, and I'd be happy to setup a call.