I'd like to make my fighter character better; my goal is to focus on tripping, but reach, more damage, and disarming would also be good improvements. He's a level 2 human, with the Improved Trip, Combat Expertise, and Exotic Weapon Proficiency (Spiked Chain) feats. We're currently restricted to the Player's Handbook, Complete Adventurer, and Complete Divine, but we'll add more books as we acquire them.
For your book restrictions, and as a new player, I strongly recommend something roughly along the lines of the classic Horizon Tripper. It’s a solid, effective build that works well, fits with what you’re already doing, and requires neither lots of books nor lots of levels.
The build doesn’t require a ton of feats; a level of fighter is part of the classic build, but only one. You have two levels, which is OK – you got a fighter bonus feat instead of Endurance as a bonus feat – but your next fighter level would get you nothing. You should not take any more fighter levels. (Note: this is a general truism in 3.5: the fighter class exists, if you are desperate for feats, but outside of a couple of very specific cases, you should only take levels in it if you are desperate for feats, and then no more than 2.)
By the way, speaking of feats, a human fighter 2 should have four feats; you only list three. There’s the feat everyone gets at 1st level, the human bonus feat, and then two fighter bonus feats. You only list Exotic Weapon Proficiency, Combat Expertise, and Improved Trip; you’re missing a feat. Combat Reflexes should be that feat.
Disarming is a problematic thing; lots of creatures don’t have things to disarm (or don’t care very much if you do), and for those who do, a locked gauntlet gives a gigantic bonus (+10) that you’re just not going to overcome. You can grab Improved Disarm if you want, but I wouldn’t, at least not until after I’d really gotten everything else I wanted.
Fighter 2/Barbarian 1
Anyway, your next level should be barbarian. Rage gets you more accurate attacks, better trip checks, and more HP. Fast Movement is minor, but might occasionally let you reposition better than you could without it. The class also comes standard with a big d12 HD, which is useful. A second level could have gotten you Improved Trip without taking Combat Expertise first, which would have been awesome for you; you can ask your DM if you can retrain, but if you can’t it’s not the end of the world.
3rd-level feat: Endurance
Your 3rd-level feat has to be Endurance. If the title of the build hasn’t tipped you off, we’re aiming to enter the horizon walker prestige class, and it requires Endurance.
Fighter 2/Barbarian 1/Ranger 2
The two next levels should be ranger. These will give you a lot of skill points, plus Knowledge (geography) as a class skill, which you’ll need for horizon walker, while you get to keep gaining +1 BAB every level.
None of the choices for combat style help you much, sadly; you could even trade them away for yet more move speed – normally a terrible trade for someone who won’t get Wild Shape, but we’re talking about something you’ll never use versus something you might occasionally use. There is a combat style in a Dragon Magazine article that grants Power Attack as the first combat style feat; that would be useful to you, if you can have it allowed without having that magazine.
As for Favored Enemy... you will most likely forget you have it. It’s a rather small bonus, that’s only going to hit a small subset of enemies you face. In a themed campaign, it could be OK, but not in most campaigns. You could try a Favored Environment instead; they’re not any better, but at least it should be a little more obvious that you can use it, and hey, it ties into the horizon walker thing pretty well.
Fighter 2/Barbarian 1/Ranger 2/Horizon Walker 10
The horizon walker prestige class is ranger-esque, with decent skills and full BAB, but it also has a quasi-magical attunement to the places it’s been. The Terrain Masteries allow you to put hard-won experience in difficult terrain to use anywhere, while the Planar Terrain Masteries allow you to tap into other planes of existence. It’s pretty cool.
The best Terrain Mastery, particularly for you, is the Desert. This makes you immune to fatigue, and downgrades exhaustion into fatigue. These are serious debuffs you’re eliminating; very useful. Better, it means you no longer have to worry about being fatigued after using Rage (though in my experience that basically never happens since Rage lasts long enough to cover the battle).
Underground is probably choice. Darkvision’s fairly useful, and in an all-human party, particularly so.
Hills and Plains make you far more aware of your surroundings, allowing you to help the party avoid being ambushed; very good choices.
Of the rest, none really stand out. Climbing and swimming are particularly niche, so I’d probably go with Forest, but you probably aren’t ever going to be stealthy...
Planar Terrain Mastery
The Shifting Planar Terrain Mastery is literally the reason horizon walker is a good class. Getting yourself dimension door once every 1d4 rounds gives you excellent mobility for a martial class. It hurts, since you cannot do much else during your turn when you use it, but with your reach and tripping prowess, position might be more important.
Cavernous is pretty awesome; tremorsense is a great detection ability.
Aligned is great if your party is spending a lot of time on the Outer Planes.
Cold and Fiery are decent pick-ups.
Weightless is kind of garbage unless your party spends a ton of time on such planes.
You get feats at 6th, 9th, 12th, and 15th. Your options for these are pretty wide open. Some picks:
Knock-down – hit somebody for 10 or more damage, get a free trip; succeed on the trip, get a free attack. Basically, against anyone standing, you get two attacks and trip as long as you don’t flub.
Power Attack – you have a two-handed weapon; it’s simple but it works. Attacks of Opportunity are always taken at your highest attack bonus and Rage gives you attack bonuses, so you should have a little room to take penalties for more damage.
Stand Still – For things you cannot trip, this feat still allows you to stop them in their tracks. You give up all your damage for doing it, but nobody is going to be making that save on anything but a nat-20.
Hold the Line – Adds a new situation in which you get to make an attack of opportunity, and this one can prevent someone from attacking you. Trip someone with this attack of opportunity, and they may not be able to complete their charge (if they lack the reach you have) or else take a big penalty for attacking while prone.
Improved Disarm – As I said, I wouldn’t bother until after the above, and then it’s kind of a ridiculous thing to take as your 18th-level feat. But then, I don’t really have any better ideas.
Actually, I kind of take it back... I really can’t find any other feats I’d particularly want from the books you have available.
The relevant resource is Tro11's The Tripping Handbook: Stay Down. To (extremely briefly) summarize;
Deformity (Tall) (Book of Vile Darkness), Inhuman Reach (Lords of Madness) - Feats
Expansion (Expanded Psionics Handbook), Enlarge Person (PHB) - Spells/Powers (wand/dorje via Use Magic Device or friendly caster (1st level apprentice will do))
Long Arm (Fiend Folio) - Graft
Wolf Berserker (Unapproachable East 45), Battle Jump (UE 42), Broken Fist Mastery I (Dragon 309), Jotunbrud (first level only, Races of Faerun 166) - Feats, such as they are.
Marshal 1 (Motivate Strength minor aura, for the bard to take) - classes
Living Chain (MiC - +2 to tripping magic spiked chain, for +2k gold add the quality to any weapon), Sweeping weapon enchant (MiC), Armbands of Might (MiC), Belt of Priestly Might (MiC), Brute Gauntlets (MiC), Torc of the Titans (MiC), Belt of the Champion (requires feat, MiC) - Magic Items
Mage Slayer (Complete Arcane), Hold the Line (Complete Warrior), Standstill (Expanded Psionics Handbook), Karmic Strike (CW)
Generally useful stuff
Knock-Down, Elusive Target (CW), Shock Trooper (CW), Spring Attack (PHB) pre-reqs.