The first place to start is Vance Dickason's "Loudspeaker Design Cookbook". After you have mastered all the material and understand the engineering behind it, you will be ready to shop for drivers.

You will find sealed boxes much more forgiving than vented ones. IMO, sealed are preferred for critical use, due to much better transient handling and lower group delay.

Inefficient speakers are not a problem here, as many have 85 SPL at 1w/1m, and this is a good listening level. Near fields won't be much farther away than 1 meter anyway.

well, that’s a tricky subject because unless you have an anechoic chamber in your basement and some other expensive high-tech gear your probably better off just spending the money for something you know is an industry standard like Yamaha or Tannoy... I bet you already knew that, so… umm… what size monitors are you trying to build? near-field, studio? There are lots of different kinds depending on your needs. Post a reply and let us know. Check out the “cook-book” bgavin was talking about and check out http://www.t-linespeakers.org/. It’s a great site for Transmission Line types of monitors. A friend recently show me Fostex’s site, they have box designs for their drivers you can download. Your going to have to study electrical theory a bit to match up your x-overs and drivers. Avoid using cheesy ms-dos freeware programs to do the calculations for you, they suck. Also read up on studio design, isolation and reverberation. Understand the basics (like phase). In my opinion this is a lot of fun if you’re going at it as a hobby but you’ll wind up spending more money for the product quality then if you just bought some, that is assuming you’re going to use them to mix on. Here are some more sites of Driver Manufacturers and Distributors.
http://www.d-s-t.com/main/index.htm (Makers of Vifa, Logic, Peerless, Scan-Speak),
http://www.madisound.com/
http://www.seas.no/
http://www.speakercity.com/
http://www.zalytron.com/
http://www.audax.fr/.
Paradigm makes good drivers too.

I've rolled my own crossovers, and won't do it again. Active crossovers are vastly superior, cleaner, and immune to impedance variance at the crossover frequencies. I'm a bi-amper.

If you are going to build you own, take the extra time to time-align your drivers with each other. This means centering their voice coils in the vertical plane to eliminate lobing and phase suckout. The better active crossovers will let you adjust this electronically.

bi-amping sure does make a world of difference in sound quality. I too once rolled some coils for x-overs I made. I wish I had those few hours of my life back... I even once tri-amped a system and added active outboard x-overs on a pair of Dunlavy SCIV's running off 6 Melos monoblock amps.

I don't have a recording studio, so my bi-amping is all done in the field. I'm the bassist for Code Blue, and bi-amp my 15" subs and JBL 10s with PLX 3002 power amps. It's fun to show up with 6,000 watts in the rack... even though it doesn't get used.

:D

In the studio, I'd bi-amp without hesitation. A sophisticated 24dB L-R type active crossover doesn't introduce near the sonic garbage in the crossover region as do the passive types. Signal coloration is never an issue, due to the crossover being immune from driver impedance issues.

IMO, a bi/tri-amped, time aligned, studio rig based on a D2 alignment sealed enclosure would be the ticket for accurate mixing. I don't understand why I see thousands of dollars spent on pres, and relative zilch spent on crappola monitors.