THE WARNER PAPERBACK Library has quietly taken a plunge into some of the murkiest waters in mass-market paperback publishing. It is a shallow plunge, to be sure, compared to the breathtaking high-dive Avon took when it issued Shanna at $3.95, but a plunge nonetheless. What Warner did was re-examine its premises after This Loving Torment had sold its first million copies (give or take the odd hundred-thousand) at $1.95 and then unobtrusively raise the price to $2.25, thereby taking a semi-advanced position (still far behind Shanna but 30 cents ahead of Rogue's Mistress, Touch Not the Cat and The Pride of the Peacock ) in the pricing of romantic paperback fiction.

Price structures in the mass market have been volatile for some time, with the more timid publishers (and those in such popular genres as romance, action-adventure and science fiction) trying to stay below $2 - which is held to constitute what stock market analysts might call a line of buyer resistance. The more venturesome (and those with titles that have already done well in hard covers - Passages, Blind Ambition ) are advancing as far as $2.50 with acceptable results, and that may be expected to emerge, sooner or later as the new base price. Meanwhile, the paperback best-seller lilsts are demonstrating that people will pay $2.75 for books which weighed more than five pounds in hardcover (Trinity, Shogun ) and for much-discussed books which are considered unique (The Hite Report ). And Bantam is currently betting that a title that is both bulky and steamy, Genius and Lust , will separate a significant number of customers from $3.50 apiece.

The new price gives This Loving Torment the same economic status as The Users. The Deep and A Man Called Intrepid, except that there may still be a few of the million-odd low-priced copies unsold. Prospective purchasers would be well advised to fish around the back of the display, and perhaps save the price of a medium-size candy bar. Back in Paper

LIsted below in no particular order are some former hardcover books whose paperback appearance you may have been awaiting.

Your Erroneous Zones, by Dr. Wayne W. Dyer (Avon, $2.25).

Genuis and Lust: A Journey Through the Major Writings of Henry Miller, by Norman Mailer (Bantam, $3.50).

The Resolution of Conflict: Constructive and Destructive Processes, by Morton Deutsch (Yale, $5.95).

Raise the Titanic, by Clive Cussler (Bantam, $2.25).

Shelley: A Critical Reading, by Earl R. Wasserman (Johns Hopkins, $5.95).

One Hundred Years of Solitude, by Gabriel Garcia Marquez (Avon, $4.95). Its unusual bulk made this sprawling novel a bit unwieldly in mass paperback format, and its classic status certainly entitles it to its new trade paperback edition.

Alistair Cooke's America (Knopf, $8.95).

The Groucho Phile: An IIlustrated Life, by Groucho Marx (Wallaby, $7.95). More than 700 illustrations, with a brief introduction and running commentary.

Metodo Cortina: Ingles en 20 Lecciones, por R. Diez de la Cortina, y revisado por Manuel Durian (Dolphin, $3.95). Those who have penetrated this far into the depths of The Washington Post presumably will not need to teach themselves English in 20 lessons (with the aid of a special record, obtainable by filling out and mailing a post card enclosed in the book). One notes with awe, however, that this is the book's 129a edicion, completamente revisada . Simultaneously issued by the same publisher for the same price are Cortina books which will help English-speakers to teach themselves French (82n edition). Spanish (154th!) and German (merely the seventy).

Two books of related interest, both bilingual, will probably fill a fairly substantial need, and their titles should be self-explanatory to those who wish to purchase them: Como Trabajar o Comunicar en un Restaruante o Bar, and Como Trabajar o Communicar en un Hotel, Motel o Posada. Both are part of a bilingual series of "how-to" books, priced at $3.95 each, and if unavailable in bookstores can be ordered by mail for $4.50 from Newhouse Press, P.O. Box 24282, Los Angeles, Calif, 90024.