I'm looking to pick up another source book or two for my campaign, but I don't remember exactly which books I already own. Is there a menu or something on D&D beyond that shows which books I have already purchased?

In the marketplace (on any page other than Featured) - there is a filter option to not display items you own (set it to off/grey/left).

If you want to know which books you've bought you can find a list in your account page: https://www.dndbeyond.com/account/marketplace and scroll to the bottom list Current Product Licenses.

Same frustration here.  It certainly seems like choosing "Sources" should at least show a little star next to the books you own.  That would make this SUCH a valuable resource during play.  It's incredibly frustrating when you choose a book you THINK you own, then you see the table of contents (Yay!), then you choose a subject and... BOOM!  Sorry pal.  You gotta buy it!  I understand that this is a classic upsell sales technique, but it's actually just bad UI/UX.

I thought it was funny when I brought this up to customer support via email and, after they told me how to see the books I haven't purchased (?), they responded "...users have advised they generally remember what they have purchased..." 

Um, okay.

It'd be nice for the desktop version of D&D Beyond had feature parity with the mobile app.

The app allows you to sort through the sourcebooks you own.

I recently found a really tricksy way of seeing what you own and what you don't.  Go to the marketplace and open the legendary bundle.  it will display a list of all the source books with what you own faded out.  

Four years later and this is still a valid question. Damn, this website is trash on so many levels. 

Still an issue? How has this not been addressed yet?

Yep still an issue...

somehow still an issue...

So this is totally a thing in the mobile app, and that's awesome. But the mobile app is TERRIBLE at reading these things - you cannot go back to your last page if you clicked a link, and the character sheets are so restricted on larger displays (like iPads) that... WTF guys?

You can obviously filter by owned items in the app. Just add it to the web. Why is that so hard?

Bumping this...add a My Library or something, please?

This has bothered me for years as well. Why isn't there a section under collections that says "my library"?? If the Devs or whomever think this helps sales by forcing people to the marketplace, you are very wrong. 

I couldn't agree more about needing a library section added to the content area!

Have you all been sending feedback requests to them via the contact area?  I just sent this...

Myself and the VAST MAJORITY of people using the DESKTOP site would like a "My Library" section added to the collections section. In this library would be all of the purchased source books and adventures.

Please consider this as the current way you have it set up is extraordinarily difficult to deal with.

I can't believe this is still an issue 2 years later.  I abhor the customer service response at this site.  It is simply ridiculous that is such an endeavor to view your PURCHASED items.  It is also almost impossible to view shared content in the campaigns section.  THIS IS RIDICULOUS.  Gte a clue.

But luckily we have the very important "my dice" section.

ROFL! This had me spitting up my soda!!

Quote from fieldmechanik >>

But luckily we have the very important "my dice" section.

 Truly the saving grace.

Bumping just to hopefully catch someone's notice.

This is prolly a tactic to drive purchases...business folks won't let devs do it. Right now, they can force folks into more frequent and higher pressure buying situations when they find just the right piece of content through general search, only to find that they don't own the source. Which is also why it's easy to find and quickly reference stuff on mobile, but not easy to read/navigate it at length. Both reinforce a different purchasing behavior.

In other words, it drives you to do the majority of your prolonged content engagement via the web, where the more content they publish, the more likely it is that a user who is deeply engaged with the content is to find and buy something that fits their need. But still provide enough value in the mobile app to allow you to use mobile for quick reference and quick on the fly purchases (individual feats for ex) during gameplay. It's a basically a dark (meaning concealed or indirect) system that simultaneously encourages meso- and micro-transactions. It's just annoying as hell.

I could just be a conspiracy theorist though...lol

still an issue

Still an issue