If you are worrying about Prince Charles and Princess Diana, stop. He is quite happy married to her and she to him. She gets on with the servants, and since the Queen enjoyed her weekly baby-sitting evening with her own children, she should be willing to take charge of her grandchildren from time to time. The Prince rarely gets angry with anyone, though tardiness irritates him. He doesn't smoke, and if anyone traveling in his car asks for permission to light up, he is told to go ahead but warned, discouragingly, that the car has no ashtrays.
All this nonvital information comes from one of the Prince's former valets, Stephen P. Barry, who has taken the trouble to put it all in the book Royal Service (Macmillan, $14.95). The fact that by publishing it, he broke the terms of his employment contract could detract from his credibility, but it won't, says Buckingham Palace, land him in legal trouble unless his memories are published in the United Kingdom.
When he writes about the Prince and the royal family, Mr. Barry is never sensational or unkind - unless it's unkind to betray the privacy of a family that cherishes it so much. He seems unaware of the irony when he tells us that ''cozy'' is a favorite praise word with royalty, and picnics (where informality and privacy rule) are a favorite pastime.
In fact, as the reader begins to comprehend how members of the royal family hunger for their rare moments of privacy and long to be ''off-stage,'' one feels guilty reading these revelations, let alone writing about them - though there seems no point in locking the stable door at this point.
''Royal Service'' is a Book-of-the-Month Club alternate selection.
''To all intensive purposes . . . I'd give my last dollar to be a millionaire . . . but what side of the fence are you straddling?''
Jack Smith relishes this kind of phrase. In fact, he's in love with words, particularly with the strange use and misuse of them; he writes a weekly column about them for the Los Angeles Times. Some of those columns are reprinted in his delightful book, How to Win a Pullet Surprise: The Pleasures and Pitfalls of our Language (as in the schoolboy's announcement, ''In 1937, Eugene O'Neill won a Pullet Surprise'') (Franklin Watts, $12.95). From this book, to be read and relished in small doses, you can discover such treasures as a schoolchild's definition of a surprise (''You can't wait and you don't know what it is but you want it''), or the meaning of some unlikely words, like ''giffgaff'' (''give and take, informal conversation'') or ''mumpsimum'' (''an error or prejudice obstinately clung to''). In fact, the whole book lends new meaning to the old phrase ''play on words.''
When it comes to the words ''bygone elegance,'' how about Agatha Christie's version of the English breakfast quoted in The Life and Crimes of Agatha Christie, by Charles Osborne (Holt, Rinehart & Winston, $17.95): '' 'Omelette,' mutters Lord Caterham as he peers under the lids of half a score of heavy silver dishes, 'eggs and bacon, kidneys, devilled bird, haddock, cold ham, cold pheasant.' '' Charles Osborne's book neatly combines biography with a taste of every one of Miss Christie's novels, plays, and poems, and a touch of criticism.
The stately home breakfast is not all that has vanished from the British scene. Judging from today's books, it would seem that we must also say goodbye to the kind of murder-without-violence, the mystery designed to puzzle, not shock, which was Agatha Christie's stock in trade. Unless, of course, we have a good public library, or access to paperback reprints - and Mr. Osborne to guide us to vintage Christie.