Apparently "in-between" refers here to the time between birth and death, so it means "making the most of life". This is based on the following WorldCat entry:

"Making the most of the in-between
Author: Elisabeth Kübler-Ross
Publisher: Head Waters, VA : EKR, Inc., 1987.
Edition/Format: Audiobook : English
Summary:
This lecture with Elisabeth Kubler-Ross discusses unconditional love and how to live fully and enjoy the in-between time between birth and death."
http://www.worldcat.org/title/making-the-most-of-the-in-betw...

The Wikipedia page and the Elisabeth Kübler-Ross foundation page both list this title among here works but give no details apart from "(Various Foreign), 1996".
http://en.wikipedia.org/wiki/Elisabeth_Kübler-Ross
http://www.ekrfoundation.org/books-2/

If it is any help, the details of the Polish edition are as follows:

"Author Kübler-Ross, Elisabeth (1926-2004).
Title Życiodajna śmierć : o życiu, śmierci i życiu po śmierci / Elisabeth Kübler-Ross ; przeł. Elisabeth Stahre-Godycka.
Other title Tyt. oryg.: Making the most of the inbetween,
Publisher Poznań : Księgarnia Św[iętego] Wojciecha, 1996."
http://karo.umk.pl/Karo2/Exec/z2w_f.pl?kl=26909-1340005372&a...

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Note added at 2 hrs (2012-06-18 07:58:07 GMT)
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So this work is a lecture, given in English, of which a sound recording exists. This recording, on cassette tape, was donated to the Library of Congress by the author in 1993.
http://catalog.loc.gov/cgi-bin/Pwebrecon.cgi?v3=2&ti=1,2&SEQ...^*&CNT=100&PID=xMDJPfgZqaRqlQBat2eGgmcSsn&SID=1

The Polish printed edition was presumably translated from this recording.

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Note added at 3 hrs (2012-06-18 08:01:48 GMT)
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Sorry, the Library of Congress link doesn't work, but you can find the full catalogue record by going here, entering "Making the Most of the In-Between" (with hyphen) in the "Quick Search" box and then following the links:
http://catalog.loc.gov/

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Note added at 4 hrs (2012-06-18 09:08:08 GMT)
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May I say that Catherine's proposal is entirely reasonable as well as propertly cautious, and it is what I thought it must mean until I found the WorldCat reference with a summary.