History 146 Readers Elementary 116 Readers (Elementary) 77 Electronic books 75 Exploration 50 Astronautics 49 Economic aspects 42 more ... Dora the Explorer Fictitious character 36 Alphabet 32 Social aspects 32 Economic conditions 28 Grammar, Comparative and general 28 Children's stories, English 27 Environmental policy 27 Environmental aspects 26 Science 26 Sustainable development 24 Economic development 23 Political aspects 23 Research 23 Board books 21 Readers 21 TECHNOLOGY & ENGINEERING 21 Geochemical prospecting 20 Philosophy 20 Animals 19 Geology 19 Mining 19 Politics and government 19 Syntax 19 Toy and movable books 18 Climatic changes 17 Economic policy 17 SCIENCE 17 Social life and customs 17 Space flight 17 Travel 17 Astronomy 16 Children's stories 16 Explorers 16 History and criticism 16 Astronauts 15 Government policy 15 Phonetics 15 Semantics 15 Space flight to the moon 15 Study and teaching 15 Study and teaching (Middle school) 15 Congresses 14 English language 14 Environmental economics 14 Manned space flight 14 Manners and customs 14 Prospecting 14 Adventure stories 13 Earth sciences 13 Economic history 13 Psychological aspects 13 Creative ability 12 Data processing 12 Description and travel 12 Education 12 Energy policy 12 Fiction 12 Insects 12 Juvenile works 12 Medicinal plants 12 Methodology 12 Petroleum 12 Phonetic method 12 Physical sciences 12 Reading 12 Seismic prospecting 12 International cooperation 11 Seismic waves 11 Weather 11 Marine animals 10 NATURE 10 Natural resources 10 Social sciences 10 Statistical methods 10 Biology 9 Environmental protection 9 Guaranteed annual income 9 Human ecology 9 Mathematical models 9 Prospection ge ochimique 9 Space stations 9 Therapeutic use 9 Commerce 8 Discovery and exploration 8 Geophysical methods 8 Mathematics 8 Plants 8 Seismic reflection method 8 Social conditions 8 Space sciences 8 Stars 8 Technological innovations 8 Trees 8 less ...