PARENTS can be duped into buying sub-standard textbooks for their children because of a loophole in quality management regulations. While publishers have to gain approval for their main textbooks from the Education Department, additional parts of a series such as work books or grammar books do not need to go through the same checks. Standards for these extras are often considerably lower than for the main textbooks. But parents, wanting to give their children as much help as possible, unwittingly buy the textbook with all the supplementary publications, which are ''not worth the paper they're printed on'', according to sources. Books for a secondary school pupil can cost more than $1,500, while parents of primary school children may need to spend $900 for a full set of textbooks. Prices are going up again this summer, with increases around 10 per cent, roughly in line with inflation rates of earlier this year. Parents who want to buy additional books for their children not listed on the official school list are being advised to check with the subject teacher to make sure it is a worthwhile investment. An Education Department spokesman said: ''All textbooks, including supplementary materials, on school book lists must be approved by the Education Department. ''Some schools may use workbooks or other books not on the list as additional exercises for pupils and they may not be checked. ''But if a certain book is not on the approved list a school cannot use it on its own list. ''It is only the supplementary materials not specified by schools that may not be checked.'' But privately, established publishers hold different views. One executive, who did not wish to be named, said standards varied greatly between the books that were approved and those that were not. ''There are some very poor textbooks around,'' he said. ''There is one that somehow got Education Department approval, but most of them are the extras - the workbooks, the grammar books and so on which aren't checked. ''You get almost rampant publishing of these extras and parents can get caught out if they don't know exactly what they are buying. ''They have to rely on teachers' advice or stick to the established publishing houses.'' Another source said there were poor quality books still in use in schools, despite the regulations, but changes were becoming more acceptable. Some years ago teachers were loath to take a title off the list, but now they were far more prepared to make changes if a book proved unsatisfactory in class work, she said. To recoup some of the hundreds of dollars spent each year on their books, students often sell them to colleagues in lower classes or second-hand book dealers at the end of each course. Last year, Director of Education Mr Dominic Wong Shing-wah hit out at publishers for making minor but frequent changes and restricted them to revising textbooks not more often than every three years. Any publisher who breached that rule could find their titles being taken off the department's recommended list, effectively barring their use in schools.