The trickiest part is getting to the final room. Once you defeat the main librarian a red portal will activate on floor taking you to a platform. Search for a rather small book to move on the nearby bookshelf ahead of you. This book will open the portal in the Secret Laboratory. To open the secret passage to the Laboratory itself you need to click the big books on the floor until one ports you up to another big book. Click that one and the secret passage will open.