I have gotten emails with this question enough times that I thought it might be worth a blog post. "What's the best way to incorporate both landscape and portrait photos into a minibook?"
I have a few ideas.
Option one : Don't worry about the direction. Make a book that's a square shape and crop all your photos to 4x4 (or 5x5, 6x6 etc). The orientation of your photos becomes irrelevant and you can pick your favorite part of the photo to fill the whole page. I love these books by Maya Road.
Option two : Edit your photos prior to printing and crop to appropriate orientation. Works perfectly to cut out background noise as well. I use iPhoto or photoshop elements to make edits but most online photo ordering places will let you adjust and make crop changes prior to purchase. (Sidenote : get 25 free 4x6 photos from Color Inc with promo code eb09)
Option three : Think ahead and only take photos with a certain orientation. I made the skeleton of my NYC book prior to leaving so I knew the photos would have to be landscape to fit the book. I kept this in mind while taking pictures and even took my polaroids sideways so I could punch holes later through the white part and not the photo.
Option four: Make the book just a little bit larger, like 6x8. Use pieces of patterned paper or cardstock as the base of the book and mount the photos on the paper leaving a white space around the 4x6 landscape photos. Two horizontal 4x6 photos will sit snugly right next to each other.
Option five : Turn the photos and let it go. Often, it's more important to get the photos and memories in the book than to worry about the fact that they are not going the correct orientation. I think this technique is the best one if you already have the photos printed and they are going all sorts of directions.
I hope these suggestions are helpful! Mini books are really the best way that I have found to get stuff documented and thoughts on to paper.
Have a happy Tuesday.