What is the difference between an interested amateur and a serious reader? I'm rather an amateur when it comes to African history, and I take the chances I get to read it very seriously.

That said, I can speak a little to the medieval section.

The books listed are academic quality: peer-reviewed, citing their sources to direct you to further information, some of them written by absolute top-notch scholars in the field, hitting quite a few of the high notes of recent and semi-recent medieval scholarship. They're not traditional-style textbooks, but some of them are frequently used that way. On the flip side, I had to read quite a few of them for my doctoral exams. For the most part, they're accessible outside a university library and not cost-prohibitive (yeah, I'm looking at you, Brill).

Some of them are a joy to read. Some of them are important but more intimidating. Some of them are more specialized to the interests of the users who added them--I might raise my eyebrows at a couple as recommendations for a non-academic audience, but I mean, that's the entire point of a wiki!

Is reading the best way to learn about the Middle Ages (your time period of interest goes here) if you don't have access to a really knowledgable historian? Of course. Is the booklist an alternative to an undergraduate class, as in a replacement? Well, that's verging on the current debate about the value of college, MOOCs, independent reading, etc., and I bet you could get some great feedback on that in r/AskAcademia.

I'll say that one of the chief weaknesses of the booklist, at least for medieval, is the lack of primary sources. You can get some great ones in solid translations off the online resources lists. But people largely aren't. AH fields questions like "What's the evidence for Jesus" and "why did Rome fall", not "um, what's with all these medieval people seeing dead babies on the altar at church?" A good teacher can help filter through the enormous wealth of sources out there, find ones that are interesting, and explain exactly why medieval people seem so excited over dead altar babies and worried about zombie priests. A good class, or a good discussion with a historian (AH plug goes here) allows you the chance to ask questions that the author of a book might not have answered, or might not have explained well enough. A class allows you to think of new questions to ask, instead of rehashing the AH FAQ. :)

But that chance is a privilege, and a rare one. In which case, Tolle lege. If you pick up a book and find it's not what suits your style, go on to the next one. Libraries are your friend! And whoever you are, the AH booklist is a great place for you to start.

There's a mix of reading levels. Most of the books would be more appropriate for the reading level of an upper-undergrad student, but there are popular materials on the list as well. Nothing I'd say is truly specialized literature only a grad student or higher would be comfortable with. The booklist is crowdsourced from the flairs and a lot of them base it on "Average-to-Higher Adult Reader" level, that's the guideline we give if they ask for one.

Is there something in particular you are hoping to do with the list?

To be really frank, the wiki book list is the sum of all that contributors (AH flaired users) have put into it over time. As such, there is no one standard that that list adheres to.

For specific topics, don't be shy to make a post here asking for recommendations after you review the book list!

I don't know about all the categories, but I glanced through the ancient Rome section and it looks like they have a really nice mix. Old classics like Syme and Brown, popular books for the general reader like Goldsworthy and Beard, and some more academic books like Eckstein and Fishwick.

It's tough to only pick a few books to list, but they have a wide range of publication dates, topics and targeted readers.