1) Comics are issued in 'story arcs'. An 'arc' is considered one publishing sequence within a story. For instance, in the Dark Tower comic series, there are four planned story arcs (I think there will be many more). The first arc is the one you purchased, The Gunslinger Born, and consisted of seven issues. The second arc, published earlier this year, The Long Road Home, consisted of five comics. The third arc, being published now (started on 9/10), Treachery, will have six issues one published each month. The next story arc is not yet defined, but should start in April or May of next year.
2) Comics are never reprinted with exactly the same cover. That is how you know which printing you are getting. So, as you have seen for Marvel comics, a 2nd or 3rd printing will have a different cover artwork. In some cases, the 2nd, 3rd or 4th printings are actually worth more than 1st printings. This is because so few may have been produced. As the publisher, in this case Marvel, figure out how many will be purchased of an issue the need for additional printings goes down. Marvel produces different cover art. Some smaller producers do not change the cover art.
3) In addition to the regular covers, there are two special covers. These are sold to dealers based on the quantity they purchase of the regular covers. One is called a 'color variant' and the other the 'sketch variant'. In the case of the Dark Tower, the color variant is normally published at a ratio of 1:25 with the regular cover. The sketch variant is published at 1:50 or 1:75. So, the color and sketch variants normally sell for much more than the regular covers.
4) Comics are sometimes signed at shows or under special agreements. These can be fairly common or fairly rare. They are generally signed by the author, the artist, the colorist and/or the letterer. The certificate of authenticity is produced by the dealer. It is accepted on faith that the signature is real and that the COA is valid.
5) There are also signed copies that are produced by either the publisher or the distributor. In the case of the Dark Tower series, many are produced by Dynamic Forces. However, just as many are signed at shows and special signing events.
6) There is a certifying agency for comics, the CGC. They grade a comic on a scale from 0 to 10 with 10 being perfect. They then encase the comic in a hard plastic container with the information about the comic and the grade inside the case. Comics below 9.8 are fairly common. Comics of 9.8 are the normal ones purchased and traded. 9.9 and 10 are rare.
7) CGC has signature series where they will get the comics signed, graded and encased. These are the highest value comics depending on grade because CGC guarantees the validity of the signature.
8) The publisher will put out special 'add ons' that are also collectable and that enhance the story. In the case of the Dark Tower, there have been some excellent ones. The Guidebook describes in detail the characters and other information about End World. The Artist's Sketchbook was published early and shows preliminary artwork by the artist Jai Lee. There are also other collectables like book-marks, promo cards and newspaper like publications with stories about the series.