Harvard library personnel have encountered serious difficulties in trying to arrest the deterioration of many books dating from the late 19th and early 20th centuries, a library consultant said yesterday.

Doris C. Freitag, consultant for conservation of library materials, said a large number of such books have had to be repaired, replaced or discarded because of their steadily worsening condition, and she added that many more face similar fates if major steps toward their preservation are not taken.

Freitag declined to give an estimate on the financial value of books that have been lost so far because of deterioration. She said that outside estimates have been too conservative, but would only describe the cost as "very high."

Books of this vintage are actually far more troublesome than those printed before the mid-1800s, when the quality of paper used took a turn for the worse, Freitag said.

She said that the libraries often lack air conditioning, humidity control and proper lighting. Freitag added that a 1976 survey conducted by the University showed that less than half of the 40 largest libraries at Harvard were air conditioned and humidity-controlled, and most others have no money to add air conditioning.

Some of the rarer books have been placed in climate-controlled storage rooms, Freitag said. The lights in these rooms are screened to keep harmful rays from reaching the books.

Some books are stored in acid-free boxes and envelopes, which "won't stop deterioration completely, but will slow it down," Freitag said.

She added that these measures are having only limited success because there is not enough rare book space. As alternatives to storage, many books have been microfilmed or photocopied.

Despite the progress she and others have made thus far, Freitag said that she fears that these efforts "may be only a drop in the bucket," adding that "sometimes I get discouraged when I see the shelves and shelves of things that should be done."