One sheet of 3/4" plywood ripped into strips 15 1/2" wide (you only need a little more than a half sheet but will need to buy a full sheet to get the cuts)
8 - 2x2 @ 8 feet long (may not need this many but added extra because of the angle cuts)
5 - 2x4 @ 8 feet long
4 - 2x4 @ 72"
10 - 2x2 @ 27 1/2"
5 - 3/4" plywood @ 15 1/2" x 27 1/2" wide
6 - 2x4 @ 15 1/2"
4 - 2x2 @ 15 1/2"
4 - 2x2 @ 33 1/8" (CUT TO FIT!!! both ends cut at 25 degrees off square)
8 - 2x2 @ 16 1/2" (CUT TO FIT!!! longest point measurement, one end cut at 25 degrees off square, other at 40 degrees off square - see diagrams)
Please read through the entire plan and all comments before beginning this project. It is also advisable to review the Getting Started Section. Take all necessary precautions to build safely and smartly. Work on a clean level surface, free of imperfections or debris. Always use straight boards. Check for square after each step. Always predrill holes before attaching with screws. Use glue with finish nails for a stronger hold. Wipe excess glue off bare wood for stained projects, as dried glue will not take stain. Be safe, have fun, and ask for help if you need it. Good luck!
Build the two face frames for the front and back. These guys should be identical.
Drill 3/4" pocket holes around all sides of the shelves. Attach shelves to the face frames. Drill 1 1/2" pocket holes on each end of the 2x4s and attach to the legs with 1 1/2" pocket hole screws. Then attach shelves to the 2x4s though the predrilled 3/4" pocket holes with 1 1/4" screws.
Make this X Bookshelf! Free step by step plans from Ana-White.com
Repeat the same for the middle shelves, but this time, use 2x2s on the ends instead of the 2x4s.
The best way to get your Xs just perfect is to take a 2x2 and hold it over where it will go. Mark the angle cuts and then cut with a jigsaw. These guys can be attached with pocket holes (1 1/2" and 2 1/2" pocket hole screws) or can be attached from top and bottom with 2 1/2" wood screws predrilled. Also attach to the middle shelves with screws or nails.
Measure and cut these guys to fit as well, as you did the larger X pieces. To attach these guys, pocket holes are probably the best option, but another alternative is good wood glue and clamping.