The top four books in this search are all guidebooks for Olympic hiking:
Olympic guidebooks at Amazon
The third one by Craig Romano would probably be my pick; I have a few other Washington hiking books by him that are really good. The Washington Trails website is helpful too. Here's link to their hike listings for Olympics:
LINK (not embeddable for some reason): http://www.wta.org/go-hiking/hikes/hike_search?title=®ion=922e688d784aa95dfb80047d2d79dcf6&subregion=all&rating=0&mileage%3Aint=0&elevationgain%3Aint=0&highpoint=&searchabletext=&sort=&show_adv=0&filter=Search
The WTA site includes the Craig Romano writeups from his Olympic guidebook (for hikes that he has in that book) plus you get trip reports from other people. You can review trip reports for hikes in May of previous years to get an idea for whether one will be doable for you.
The Trails Illustrated map is also good resource:
Trails Illustrated ONP map
Although you can download less detailed ones from NPS website:
You can also get hiking info from rangers at any of the Wilderness Information Centers. The main one at Port Angeles/Hurricane Ridge also sells all the books and maps listed above, I think. But call them for more info; they'll be helpful:
ONP Wilderness Info Centers
As Ozzy and Dale W. already said, nwhikers.net is excellent resource. May stands good chance of being cool and rainy, especially on west side of park. Snow could be issue precluding many hikes, but the river hikes on west side of park should be good. Hoh River is always good, and I would personally suggest doing a day hike partway up the Enchanted Valley Trail down near Lake Quinault in southwestern corner of park. Quinault Lodge near there is part of NPS and there's a Wilderness Info Center a few steps away, which might(?) be open in May. Also, the hike up the Elwha River (right outside Port Angeles) will be easily doable in May (can do it almost year round); I did that towards end of May last year).
Also, not sure why someone said paid guides are illegal in ONP. Here's link to a commercial guide's site that provides guides for "groups of eight or more".:
Guide for ONP
I know of another outfit that does guided multi-night backpacking expeditions in ONP.