I am tiling a small bathroom ~45sq ft. The book I am referencing suggested starting the tile in the center of the room to prevent laying any tiny fractions of tile near one side of the room. I'm working with 12x24" tiles and it looks like I might be better suited with starting the tiles at the door threshold. Is the start at the middle idea just a guideline that I can go ahead and break?
The two things (starting in the middle and avoiding slivers) are NOT related.
You can start in the middle and end up with idiotic cuts on both edges - or you can start in the corner and not do that...
What you SHOULD do is measure enough that you know where to put the SENSIBLE cuts on the edges, regardless if you are starting at a wall, corner, or the middle of the floor. When starting in a corner or at a wall, your very first tile might well be a cut tile. By planning, you know what cuts you are making and can place the whole and cut tiles as you like. In your case, the first two-and-a bit tiles at the door should probably be cut so that there's not a large gap at the threshold (or you need to get the door molding cut so they can slide under it.) Depending how the (invisible) far wall (opposite the door) comes out, you might want to take enough off the "door" tiles that the wall parallel to the door does not have what looks to be a 1-1/2" slice running along it.
The main argument I've seen for starting in the middle is to avoid following an irregular wall - but you can do the same thing by striking a chalk-line where the first grout line from the wall would be, and pre-cutting all the tiles for that wall if it's irregular (or if they will be half or 1/3 tiles so the far wall isn't slivers - which you'll know because you measure it.)
Based on your pictures, I'd choose the "not centered" approach, since the visible far wall is decidedly not slivers that way, but that's purely an opinion call, not "right" or "wrong."
Yes, starting in the middle of the room can be a bit un-nerving. There's no reason not to begin in another location ; as long as you're comfortable with the appearance of cut tiles along the wall. For the sake of balance and uniformity It would be wise (as you noted) to begin at the most trafficked entry point working out and away into the room. I always found you get the best look from the tile if you can install full tiles to be in view when the door is open. So looking at photo #2 you see more full tiles and fewer cut tiles. This will also produce less work in that you won't be going back to the tile saw constantly and there won't be as much waste. You might want to trim the door molding so the tile can fit underneath rather than notching angled cuts. Keep your spacing along the wall as close to the joint spacing as possible if you don't intend to install wall molding (it will look neater). Any grout joint over 3/16 inches you will want to use sanded grout for a stronger grout joint.
When I tiles three adjoining rooms I wanted straight lines connecting the rooms. I didn't want the dining room to be half a tile left or right of the kitchen.
I started in the middle of the family room because it was the biggest and what would be the biggest area where any mistakes would be the most visible. Then, I laid out tiles from that center one, using the spacers, into each of the rooms BEFORE I ever started mudding. Then I shifted that center tile as needed so I had as few cuts on each wall and still had my continuous lines. It was a sort of test lay.
I think another reason you start in the middle is to divide any misalignment equally into each corner. If you start in one corner and that tile is four degrees off then all of that error will end up in the opposite corner. Start in the center and that error ends up being less obvious. I don't actually know if that's true and I haven't put a lot of time into working that out. Maybe someone can tell me if I'm not even close on that one.
I have laid tile in maybe 100 bathrooms of this size. You start at the door wall and work your way out. Other decisions are based on where your vanity will be and where most of the traffic will be. On yours it looks like the area running straight out from the door will have the most tile traffic so I will use that fact in my answer.
first install your threshold where you want it and parallel to the door wall. If you do a really good job of installing your threshold it will sit right at a point where you can tile straight across without cuts and your grout width next to the threshold will be uniform.
You have a pattern. You want to offset your tiles using a horizontal pattern. In your "not centered" picture this is the way you want to do it EXCEPT do not line your first tile with the upper left hand corner (of the pic), it should be lined up with the door wall. This might not be true for a large room but surely is for a small bathroom as this area will be 80% of the focus.
to line up the tile with the door wall you will have to cut out the bottom of each of your door jambs - do not cut tile around this, as it is hard and ends up looking tacky. This is a bit tricky because you want it snug as possible but you have to account for both tile and thinset. I usually just add 1/4" to tile height.
The only time I would ever start in the middle of a small room like this would be if I was running some diagonal pattern. Since you aren't then this is moot.
I would keep your tiles snug to the left wall (in the pics). This wall is in front of the door and is the eye line for the bathroom. Any odd/non-uniform cuts I would put on the other wall.
LOL, that is funny. Might want to burn that book because any tile setter that wrote that is either a complete amateur or an idiot.
There are two basic problems with tile geometry: the room is not square, and the dimensions of the room are not evenly divisible by the dimensions of the tile. Solving these two problems is not trivial, even for experienced tile setters and for an amateur it is very difficult.
I would strongly suggest getting tile setting software. This software will let you experiment with different layouts and help you determine the best configuration for your space.
In general, what you want is for everything to be balanced, which means that the angles and cutoffs are the same on all sides. For example, if the tile makes a 5-degree angle with the wall on the left, then it should make a 5-degree angle with the wall on right. Having a 3-degree angle on one side and an 8-degree angle on the other is bad. Likewise, the cutoffs should be balanced. If you have a 5-inch cutoff on one side, then you should have a 5-inch cutoff on the other. Having no cutoff on one side, and a 10-inch cutoff on the other is bad because it is not balanced.