Yes they are actually. It really draws you into the world of halo almost as if it were the game itself. I found it quite accessible for, at the time, I hadn't played the games. It really showed me the scope of the world and familiarized me with the Characters and Enemies.
I would note that if you aren't comfortable with War style fiction and names, it might not be as accessible to a broad genre of readers. However, if you are looking for some true war style sci-fi, read dem books!!
I've read several Halo books, but can't remember the authors name. It did help to have visualisations from the game of weapons, craft and opponents. But then it doesn't take much to google up images and gameplay videos of them when you come across them, even if just to refresh your memory.
Overall I didn't think that much of the novels I read though. They were very simplistic in their writing, although that's mainly due to the target audiences needs I guess. Almost like film tie ins. So, whether they are worth the investment as a general reader I don't know. I prefer Dan Abnett's Warhammer 40k novels which are in some ways similar writing. I almost bought a three book set of Kevin J Anderson's I found in a secondhand shop the other day till I flicked through looking at the writing style and remembered why I don't read his books anymore
I will admit that the more targeted audiences keeps it from being liked by everyone but I thought the writings were very nice. I think they might be worth a tiny investment. Oh and the author is Eric Nylund unless you are reading something different.
Ok....now I must know about these Warhammer books for I am intrigued...damn you.