1. Is it true that copyright law allows me to photocopy, for example, not more than 10% of a book?
No. Photocopying a book without the licence or permission of the copyright owner is prima facie (apparently) an infringement, and liability will arise if you have photocopied a substantial part of the book. The test for substantiality is qualitative but not quantitative. There is no such rule which says that one is safe if one photocopies less than 10% (or any other percentage) of a book. It is possible that 10% of a book is already a substantial part of the book. Thus unless you have a defence (such as fair dealing) or obtained the permission from the copyright owner, you are likely to be liable for infringing the copyright in the book.