A major source of information about Maison Ikkoku on the Web is The Maison Ikkoku Guidebook, the result of much work by dedicated fans over a period of years, mostly on the MI mailing lists.
There are, unfortunately, two problems with the Guidebook. One is simply that it was compiled, for the most part, before HTML became standard, and it is not HTMLized at all: it is, rather, several huge files of plain ASCII text, without links, without color coding, without any way to access quickly the information you seek, other than by old-fashioned word search.
The other problem is that the fans who put it together were huge fans of the anime, and treated anime and manga more or less as a single whole work. I find this absurd, and, in the long run, rather aggravating. It's a little as if one went to a web page looking for information on Kipling's "Jungle Book" and found it to be full of references to the Walt Disney movie ("...and in the movie, the king of the monkeys is named Louie...").
The MI manga and anime are different works, in different media, by entirely different people, and they should be treated as such.
Nonetheless, when they did discuss the manga, they came up with lots of interesting information (of course, reader discretion is advised, as everywhere on the Web, to distinguish true information from simple fan speculation!). Most of it is on the files mi-guide-r8b and mi-guide-r8c, which you may consider downloading and perusing at your leisure. They also produced some "deluxe" translations of selected manga episodes which have lots of interesting tidbits and may be of interest to students of Japanese as well. You can find some here and a few more here (in this last site, ignore the small files, go for the large ones).
I have not made use of any of this information here (not consciously, at any rate), but I might someday, if I have the time, go over all of this carefully and point out, in my notes to the chapters, where there is some piece of information in the Guidebook that is relevant to that chapter; if I do, I will also provide a way to search for it (e.g., the appropriate keyword for a text search)--and, of course, the name of the person to whom the credit for the observation should be given. But that is a project for another time.
A detailed comparison between manga and anime can be found in Wei-Hwa Huang's site, as well as some other stuff, such as a full discussion of the names of the characters. Here also anime and manga are considered together, so one gets the occasional equivalent of the King Louie example above, but some of the information is really interesting nonetheless: I found this to be especially the case with the entry about the Kujo family.
Finally, if you can read Italian (and even if you don't) you will find much material not available elsewhere in the Italian version of the Maison Ikkoku Guide .
Many thanks go to all the members of the it.fan.r-takahashi (now it.fan.japan.r-takahashi), for lots of information and wonderful discussions, as well as to all the people who have written to express support and appreciation over the years.
After all this talk about the anime you may be wondering if you should give it a try. The answer depends on whether you like anime in general; whether you like adaptations in general (or whether, instead, you are like me, and have the tendency to leave the theater frothing at the mouth about what they have done to your favorite book); and whether you generally like fanfiction, or rather feel, like I do, that once an author is done with her characters, if anybody else ever touches them again they automatically become different characters.
Personally, I like the Disney version of the Jungle Book precisely because it is so different from the original stories, that it does not get in the way of my enjoying them at all; but the Maison Ikkoku anime is just similar enough to the original story to make all the differences really annoying, and I have come to the point where I am no longer capable of watching it at all.
Your mileage may vary, however. Taken in itself, it is certainly a fine piece of animation. If nothing else, you may want to try watching the opening and closing episode trailers, some of which are extremely cute (sometimes straight out of Takahashi's drawings). They are available here. (Or try getting them from here, if those other links don't work.)