Option L on the login menu is a program called 'Library'. This service is MUD's data bank, a mass of information about the game written mainly by the players.
Library consists of a number of files, organised into units themselves called 'libraries'. On your screen, the contents of a library will be listed in menu form, and you simply type in the number of the entry you wish to take. There are several different sorts of entry, but the main three are libraries, data files and boards.
Since a library can have other libraries as entries within it, this gives an hierarchical, tree-like structure overall. All files have an address which gives the route to that file from the uppermost library, separated by dots. For example, entry 4 in the library which is entry 1 from the uppermost library would have the address .1.4 . The library at .3 contains a whole host of other libraries managed by some of the wizards and witches of MUD - the librarians - and each has its own individual flavour.
Those entries which are not themselves libraries are, in the main, data files. The library contains many, many of these covering virtually every aspect of MUD, and newcomers are strongly recommended to browse through them. Although not all will be immediately relevant, you'll soon get a good impression of where it's best to look. As there are lots of librarians, you should be able to get a feel for which library has the style you prefer, and contains the sort of information you find most useful. The library at .4 contains MUD's (usually monthly) on-line magazines, which are always well worth reading.
There are other types of entries, 'boards' being the most prominent. These are proper bulletin-boards, where messages can be posted for public consumption. The main board is at address .5 (ie. select 5 when you first get into Library), but there are other boards scattered around in the libraries lower down in the tree, each with their own subject matter and each managed by a librarian. The board at address .2 is the MAIL board, which is set up to be used for persona-to-persona messages; it can also be accessed by taking option M from the login menu. Although most players read boards, not so many seem to contribute; please don't be shy, if you want to comment on something you've read on a board, do so!
As with all MUD's facilities, Library has comprehensive on-line help available - just type H and sit back!