A couple of things:
* In Monte Cook's discussions of Prestige Classes, he distinctly suggests that Class and/or Character Level not be used as a pre-requisite. The point here is that very few people would identify themselves as having a class and nobody would actually be able to judge what "level" they were in that class. Thus, the pre-requisites should always reflect that which is measurable: skills (tests can be given), abilities (one can test strength, dexterity, constitution and, in some cases, even intelligence, wisdom and --to a degree- charisma), feats (either you can do it or you cannot do it). Thus, I would suggest replacing the "5th level character" prerequisite with something else. A set of skills, traits or what-have-you...
* Since a game-master must approve any use of aprestige class, I would re-word a few of the pre-requisites such that this is not restated over and over. Simple state (for example) that, as a general rule, Telepaths are not allowed to be Rangers (although some extreme exceptions do exist).
* Advancement Restriction: although I like this, in principle, the mechaic is a bit clunky. I would suggest placing something akin to the Paladin's Code of Honor in here -- describe what a Ranger must act like. Then, indicate that if the character violates this Code of Honor, then the character cannot advance in this class until they have first advanced in another class. Something to that effect.
* A denn'bok is a quarter-staff. Granted, the weapon may be exotic (in the sense that it is cool how it folds in on itself); but its _use_ follows the patterns of a quarterstaff, and such the use of the weapon should not require an exotic weapon feat. It would be sufficiant to say that only Rangers and a few select Minbari of the Warrior caste have them...
* 5-levels: I am torn in this. The Rangers are a dedicated bunch; as such, 10 levels might be more appropriate. Again, citing Monte Cook, a 5-level prestige class should (1) be one that indicates a specialization in training. Generally, the individual would not identify themselves as being this prestige class, just having the training. a 10-level prestige class is one that requires a lot of training and disciplin and characters would identify themselves as being that prestige class. In other words, Marcus _is_ a Ranger -- thus, according to the words of Monte Cook, Ranger should be a 10-level Prestige class. Still, I am not sure what else you would want to add to the class, or how far you would want to spread the abilities out.
(1) Please keep in mind that even Monte has broken this rule, so it is not a big deal if Mongoose continues that trend. It just happens to be a rule I like.
Overall: Very nice. I like it.