Many of the trails are suitable for the occasional dry weekend in the winter months as well. Sullivan provides many low elevation trails which extend the hiking season year round.
If you live in southwestern Washington or northwestern Oregon, you should own this book.
The trail descriptions are both concise and crystal clear. Sullivan does not meticulously describe each detail, which leaves much to discover on your own and allows for a more personal hiking experience. He writes eloquently, and often includes great tidbits of scientific and historical information that add context to your hike. For example, he might briefly describe how a specific area was geologically formed, and point out some evidence that you'll find along trail.
Sullivan's practical advice is invaluable. If he tells you that a trail passes poison oak, you better wear long pants! If he tells you that a trail is open from July to October, don't show up without snowshoes in March! Clearly, he knows these trails well. Readers should not overlook the preface, where he provides an equipment checklist, low impact camping guidelines, contacts for trail conditions, and other useful information.
The one downside of this guide is that so many people use it. If a hike is within an hour of Portland, and is featured in this book, you can count on a crowded trailhead on a nice weekend. Plenty of the featured hikes are off the beaten path, and still consistently provide solitude. Just plan on driving for awhile.
Anybody who hikes in Oregon should consult "100 Hikes in Northwest Oregon," or one of Sullivan's other books. I also frequently use his Oregon Coast and Central Oregon guides, and they too are first rate.
Just a quick additional note: When you do hike, always leave no trace! If you see trash, pick it up! Always be respectful of our Earth, and all of its creatures.