As a missionary, I consistently carried and read from the “White Handbook”. To my knowledge this is still standard practice. I’m not sure when it was first introduced; however, I believe that the grand progenitor of our current missionary code is The Elders’ Reference, a 64 page book published in 1913.
According to the Improvement Era (1913) vol 16:
It contains a sermon delivered by President Joseph F. Smith directed to elders going on missions, and containing practical suggestions to them, also a series of practical notes by President Francis M. Lyman of the Quorum of Twelve Apostles to be referred to daily by missionaries. There are several pages of practical instructions to the missionaries on the methods of performing their work…There are also daily hints to missionaries, and counsel to returning missionaries, by President Joseph F. Smith. The book concludes with a fourteen or fifteen page appendix, containing important sayings of the Prophet Joseph Smith. It is printed in handy form for pocket use.
I was only able to track down the Notes to be Referred to Daily by Missionaries, written by Francis M. Lyman. Fortuitously, this is the portion of the book that has the greatest equivalence to the current White Handbook (the rest being more analogous to a missionary guide). It consists of 39 enumerated rules, the full text of which is available here. The following selection are notable:
3. Invariably keep the Word of Wisdom, refraining from the use of tea, coffee, tobacco and intoxicating drinks.
4. Guard against familiarity with womankind. There must be no sparking [the modern equivalent of sparking is flirting], kissing, or embracing of woman – your kisses should be for home consumption, and be brought home to your loved ones, where they belong. Kissing and hugging aside from this lead to immorality, and a fallen brother not only crucifies himself, but brings misery and woe to the kindred of both parties. Immorality is the bane of missionary life. There is little more enticing, and nothing more dangerous and deadly.
7. Bless, but do not curse.
12. Hold sacred and do not use commonly such names as God, Jesus Christ, The Holy Ghost, Apostle, Prophet, Seer and Revelator. Elder or Brother are the common titles for members of the Melchizedek Priesthood. President and Bishop may be used where they belong.
18. Leave your visiting and sight-seeing until your mission is completed.
20. Be careful of what money you may have; see that you do not get robbed.
22. Do not make promises to write or do other favors when you get home; wait until you get home, and then do all you reasonably can.
23. Do not praise the beauties of Zion, or magnify the virtues of the Saints. Fortify the people for the trials they must meet, as they will be tried in the furnace. Urge the people to stay and maintain the work abroad in the earth, by their works and their means. Thus they will gain strength to be able to stand when they do gather to Zion. If they must apostatize, it is better that they do so in their native land.
34. Do not enter into debates with each other or with anyone else over obscure points and passages; nor should you seek to advance beyond what the Lord has revealed.
37. Be punctual, that the Spirit of the Lord may not be grieved by the unseemliness of tardy attendance.
I guess things really haven’t changed that much in the last 100 years.