Hi Marianna.... - welcome to the forum! I'm assuming that you want to 'delete' books from your iPad (of course, if you sync your device w/ a computer and/or use a cloud service, then the books may still be stored in those locations).
But in 'iBooks', select the 'collection' where the book is located; if in 'icon mode', just press 'Edit', select the book(s) you want to delete and hit the red delete button (upper left); if in 'list mode' just swipe the book(s) right or left - a red delete option will appear to remove the book. Good luck & let us know if this answers your question - Dave
The IPad Forum is a community of members from all walks of life and from all over the world coming together to share our experiences and to help each other learn about this wonderful piece of technology the Ipad. There is a wealth of information already here in the many threads we have so doing some searches will answer most of the questions you have. If then you cannot find the answer you are seeking then by all means post your question. There are many friendly and informed members here only too willing to help you.