Someone asked for a breakdown as to how Ospreys work, well here tis my attempt,
Men at Arms, started in early 70s and originally focused on a particular regiment or group of regiments or on a campaign. Notable titles included the Gurkhas, the Argyll and Sutherlands, the SAS, etc and others such as The Army of the Potomac. Today they tend to be fairly subject narrow and are the cheapest in the whole range. The range has diversified recently with a whole series on strategy and tactics.
Vanguard series worked alongside the MAA series but focus on vehicles eg the one on the Churchill tank. These meta morphed into the New Vanguard series with the same function and number of colour plates (8). There are also air vanguards too.
The first new series were the Elite series this was in the mid 80s with some of the first covering NATO forces, Warsaw Pact and the Armada as a special in 1988.
This series were followed a few years later by the following,
Order of Battle, these are fairly dry unless you like statistics or do wargames. In fact if you are a gamer you should be using the Nafziger collection instead...
Campaign series (from the early 1990s)
Essential Histories these are thicker tomes but priced rather steeply.
Raids (fairly new series)
Dual (eg Firefly v Tiger)
Aces
Fortifications (I have one on Norman castles and due to nature of the subject its a short synopsis but adequate enough)
Weapons (I have the one on Medieval handguns)
If you want to buy some then go to a wargames show as they tend to be a damn sight cheaper then! The Cold War Central Front Tank Battle Elite book originally about £9.99 is now OOP and usually fetches £35 so if you find it for less, grab it!