Of course, The Book of Marvelous Magic wasn't just written by Gary Gygax. He shares the cover byline with Frank Mentzer and the dedication (written by Mentzer) thanks a large number of people "for their contributions," among them Mark Acres, David Cook, Tracy Hickman, Doug Niles, Penny Petticord, John Pickens, Gali Sanchez, Carl Smith, Garry Spiegle, and Skip Williams. Who contributed what is never delineated in the book, but my suspicion is that Mentzer wrote the bulk of the book, using his own ideas and those offered to him by others, including Gygax. Gary's name is on the cover precisely because, even in 1985, it was still a draw to players like me, even though his contributions were probably minimal (more knowledgeable souls can correct me if I am mistaken).
The Book of Marvelous Magic is a 80-page softcover book that details over 500 new magic items for use with Dungeons & Dragons (and AD&D -- more on that in a bit). These items are broken up into fourteen categories: amusements, animal-related items, apparel, cloth and related items, containers, foodstuffs, furniture, household items, jewelry and valuables, music instruments, oddities, paper and related items, tools and hardware, and travel items. Each of these categories has one or more percentile tables associated with it, to aid the referee in randomly rolling treasure. D&D magic items from the rulebooks are also included on these tables, which is nice.
The items presented in this book would qualify as "miscellaneous magic items" and are a mixed bag. Some are extraordinarily mundane, while others are bizarre. Some have punnish or esoteric names (both suggesting Gygaxian involvement). The vast majority, though, are interesting enough, if only in that they're new. One of the problems of playing D&D with long-time players is that it's hard for them to get excited about a bag of holding or gauntlets of ogre power anymore, since, to them, they're "standard" parts of the game. Having a larger pool of magic items to draw upon is thus always a good thing and I suspect a big part of the appeal of books like Unearthed Arcana to many gamers.
Over the years, my opinion of The Book of Marvelous Magic has been inconstant. After my initial excitement, I felt deflated, in large part because so many of the book's items simply reproduce the effects of a spell or class/race ability, while others are merely magical replacements for technology. Later, I came to appreciate the variety it offered, even if many of its offerings are less than inspired. Nowadays, I look on it more kindly, seeing it primarily as a fount of ideas from which I can steal -- and steal from it I have.
The book concludes with an appendix, where rules and guidelines are given for using it with AD&D. Re-reading it today is interesting.
The D&D and AD&D games are actually different games. Though both are role-playing games dealing with fantasy topics, many of the games' systems are entirely different. Each game contains spells, monsters, and other elements not found in the other.Back in 1985, I'd have seen this section as a vindication of my preference for AD&D over D&D, whereas nowadays, it's just the opposite.
The D&D game is easy to modify to your individual taste. When revised and expanded (editions published in or after 1983), some details were added, but the game as a whole actually became easier to modify. You may add more details, or change existing ones, with little fear of upsetting the game system as a whole. Options are often mentioned, giving the DM a choice of styles or details. For example, a monster's poison can be deadly, but guidelines are given for the DM who wants to change this to points of damage (or other effects) to make the game more enjoyable.
The AD&D game system is, as a whole, far more complex than the D&D game. Rules are given for more situations, and common situations are presented in more detail. Since the AD&D game is more complex than the D&D game, it is very difficult to modify properly. Any rule change may have far-reaching effects. Modifications usually involve very minor details, and rarely (if ever) change general principles. Additions must be compatible with existing details, and must thus be very carefully considered.