The Lord had commanded Joseph Smith to reveal the manuscripts to no one who was not involved in the translation. Martin petitioned Joseph three times to show the manuscripts to his (Martin's) wife, and each time Joseph took the issue to the Lord.
The Harris family was wealthy, and Martin was helping pay for the publication of the Book of Mormon. His wife protested and wanted proof of the translation because of his involvement and probably because of the financial burden. Incidentally, she badgered him on the issue and demanded to see the plates -- or something convincing.
In response to Joseph's first two pleas, the Lord denied the request, and I believe He even said to trouble Him no more on the matter (See near the end of D&C 5). The third time, the Lord finally permitted it (kind of like the Parable of the Importunate Widow) -- but not without warning of the consequences for disobedience.
Joseph conceded to let Martin take the raw manuscript (but not the actual plates), being the 116 pages that had been translated. Later, Martin returned to Joseph and admitted that he had lost the pages. Joseph lost his gift to translate and was commanded to repent:
5 Behold, you have been entrusted with these things, but how strict
were your commandments; and remember also the promises which were made
to you, if you did not transgress them.
...
10 But remember, God is merciful; therefore, repent of that which
thou hast done which is contrary to the commandment which I gave you,
and thou art still chosen, and art again called to the work;
11 Except thou do this, thou shalt be delivered up and become as
other men, and have no more gift.
The pages consisted of what would be called, probably, the "Book of Lehi" -- writings at the beginning of the plates by Nephi's father, the same Nephi whose writing now starts the Book of Mormon record as we have it today.
In the Book of Mormon, we learn that Nephi was inspired to re-write at least some of what his father wrote, so that nothing too important was lost (see vs. 1-3 in linked scriptures for more context) -- emphasized parenthesis my own:
3 And after I had made these plates by way of commandment, I, Nephi,
received a commandment that the ministry and the prophecies, the more
plain and precious parts of them (Lehi's writings), should be written upon these plates;
and that the things which were written should be kept for the
instruction of my people, who should possess the land, and also for
other wise purposes, which purposes are known unto the Lord.
Now, the writings weren't so sacred that nobody could know what they said, but rather, they had to be carefully handled -- and understandably so. Imagine the frenzy if the translation of a record on "gold plates" was public while it was happening... especially during a major Christian excitement in the country!
So, the Lord warned Joseph to not re-translate the missing portion because evil men were plotting against them. From Doctrine and Covenants 10 (chapter heading):
Revelation given to Joseph Smith the Prophet, at Harmony,
Pennsylvania, in the summer of 1828 (see History of the Church,
1:20–23). Herein the Lord informs Joseph of alterations made by wicked
men in the 116 manuscript pages from the translation of the book of
Lehi, in the Book of Mormon. These manuscript pages had been lost from
the possession of Martin Harris, to whom the sheets had been
temporarily entrusted. (See the heading to section 3.) The evil design
was to await the expected retranslation of the matter covered by the
stolen pages and then to discredit the translator by showing
discrepancies created by the alterations. That this wicked purpose had
been conceived by the evil one and was known to the Lord even while
Mormon, the ancient Nephite historian, was making his abridgment of
the accumulated plates, is shown in the Book of Mormon (see Words of
Mormon 1:3–7).
The whole half of the 10th section is relevant here, but in particular:
10 And, behold, Satan hath put it into their hearts to alter the words
which you have caused to be written, or which you have translated,
which have gone out of your hands.
11 And behold, I say unto you, that because they have altered the
words, they read contrary from that which you translated and caused to
be written;
12 And, on this wise, the devil has sought to lay a cunning plan,
that he may destroy this work;
13 For he hath put into their hearts to do this, that by lying they
may say they have caught you in the words which you have pretended to
translate.
14 Verily, I say unto you, that I will not suffer that Satan shall
accomplish his evil design in this thing.
Summary
In conclusion: The writings of Lehi were about as sacred as the rest of the Book of Mormon record, but it was vital that the work remain protected against those conspiring against it. And yes, Joseph and Martin both should have obeyed the Lord in this instance -- the first time. But the Lord saw it coming, so He prepared a way for us to still have the important parts of what Lehi wrote and allowed Joseph to repent and continue the translation (with a different scribe -- but Martin Harris still sacrificed in many ways, after losing the manuscript, to support the publication of the Book of Mormon and the Church).
