UR is Underground Rivers
FI is Floating Islands

UR -- https://play.google.com/store/books/details/Richard_Heggen_Underground_Rivers?id=TfBlDwAAQBAJ
FI -- https://play.google.com/store/books/details/Richard_Heggen_Floating_Islands?id=UfBlDwAAQBAJ

UR -- https://drive.google.com/file/d/1LjJTKW5xvFgZ92RcfRC4BhL7ss86xziv/view?usp=sharing
FI -- https://drive.google.com/file/d/1VBSEDZ3dQnwiadk_ewsZjucz6yrpauCg/view?usp=sharing

UR -- https://www.scribd.com/document/384440449/Underground-Rivers
FI -- https://www.scribd.com/document/384440464/Floating-Islands

UR -- https://ia601504.us.archive.org/10/items/UndergroundRivers_201807/Underground%20Rivers.pdf
FI -- https://ia601502.us.archive.org/20/items/FloatingIslands_201807/Floating%20Islands.pdf

UR -- https://www.icloud.com/iclouddrive/0RHl7tM-t3ssmlw-gfvQH5nGg#UndergroundRivers.pdf
FI -- https://www.icloud.com/iclouddrive/0Xty164TScIym2GuZ_C9MzPDA#Floating_Islands.pdf

UR -- https://www.dropbox.com/s/d9f36zjj9va0p3h/Underground%20Rivers.pdf?dl=0
FI -- https://www.dropbox.com/s/qhcj1h93odul66i/Floating%20Islands.pdf?dl=0

UR -- https://www.icloud.com/iclouddrive/0hz5oa_2lX2L49G-4FK03SVnQ#Underground_Rivers.pdf
FI -- https://www.icloud.com/iclouddrive/0rwaPTq4Q2E8CT2BRmDXrTW3A#Floating_Islands.pdf

UR -- https://docs.zoho.com/file/2ti8y91e967259917436faecbfbf9c9aedce7
FI -- https://docs.zoho.com/file/2ti8y84fddb217ae8480ba5971ed1110ce746

UR -- https://app.box.com/s/w34c7c8q99wb1fe4p2p3gc4o4fr7nrou
FI --  https://app.box.com/s/kifndi6l2otjw8ba2z18lvsyshv9mg8w

UR -- https://jmp.sh/v/DIywVysNmocBQrIcU23l
FI -- https://jmp.sh/v/jJjPJsanqvgavEGGkMZE

UR -- http://www.mediafire.com/file/ic3wm6mi51vwjb5/Underground%20Rivers.pdf
FI -- http://www.mediafire.com/file/bphpxbz1dusckp3/Floating%20Islands.pdf

An Internet search may reveal others, but be wary of old versions.